The "Grey's Anatomy" doctors are definitely in.

Viewers think the sexy surgeons are the latest in TV cool, while ratings have been red-hot for the ABC medical drama, which ranks fifth in viewers (18.4 million) in its first full season.

Launched in March, "Grey's" inherited the huge lead-in audience of "Desperate Housewives," but it soon attracted its own following. "There's something for everyone," says Patrick Dempsey, who plays neurosurgeon Derek Shepherd, known as Dr. McDreamy to "Grey's" female doctors.

"There's such a diverse cast that people can identify with, archetypes that people can relate to," Dempsey says. "There's an emotion to it. There's a sense of humor to it."

"Grey's" secret might be in the mix: of work and play; of comedy, drama and a little soap; of men and women; and of interns, residents and attending physicians of varied background, race and personality.

"This is essentially a workplace romance show, as opposed to a pure medical show or a pure relationship show. It's really about a group of friends trying to make it through everyday work and relationships," says creator and executive producer Shonda Rhimes.

And don't forget the sex factor. "Everybody has sex. ... Well, not everybody, but there's a lot of sex," Rhimes says.

Since more people are watching, the series' growth spurt deserves examination, with diagnoses courtesy of "Grey's" actors, producers and viewers.

CHARACTER COUNTS

Friends, as Rhimes calls them, is the operative term. Fans appear to have bonded quickly with "Grey's" cast. Viewers enjoy the stories, but they come back for Meredith Grey (Ellen Pompeo), her sometimes lover, Shepherd, and colleagues.

"The characters are so convincing, and the chemistry is electric," says Tracy Gallo of Lisbon, Ohio. In a recent episode, "I could feel McDreamy's breath on the back of my own neck during the elevator scene with Meredith."

Meredith, the "Grey" of the title, is one of five first-year interns. The show's narration comes from her perspective, but Rhimes says it "absolutely is an ensemble show."

Critics praise the rich detail of the characters, especially in a series now featuring 10 regulars with the addition of Dr. Addison Shepherd (Kate Walsh), Derek's wife.

MOVING TARGETS

Both characters and relationships are ever-changing. The early take on George O'Malley (T.R. Knight): indecisive intern. He has evolved into a talented doctor with a backbone. At the same time, cocky Alex Karev (Justin Chambers) is dealing with an uncertainty of his medical future and his tentative love connection with Isobel "Izzie" Stevens (Katherine Heigl).

Tough-as-nails resident Miranda Bailey (Chandra Wilson), known as The Nazi, could have been a one-dimensional person. But subsequent episodes have shown a broader character.

Now she's pregnant, which tests her ambition, says Wilson, who this month gave birth to her third child. "We'll see how together she is."

ONE VISION

Series TV newcomer Rhimes, who wrote "Introducing Dorothy Dandridge" and "Princess Diaries 2: A Royal Engagement," has been given broad leeway to create the world of Seattle Grace Hospital.

"Shonda has a very clear idea of who the characters are, and she writes them that way," fellow executive producer James Parriott says. Rhimes' concept combined the universality of workplace relationships with a heightened life-or-death aspect that few face. It didn't hurt that she is a big fan of surgery documentaries on cable TV.

"I know what's going to happen in Season 4, should we make it that far," Rhimes says.

TRAGEDY, MEET COMEDY

Heigl says viewers relate to the show's humor. "And I think Shonda has the wickedest sense of humor," she says.

In one episode, Seattle Grace encountered the serious and not-so-serious aspects of a syphilis outbreak among the medical staff. In another, doctors discovered a patient had consumed doll heads.

During a scene shot this week for an upcoming episode, Meredith faced the serious and the slapstick simultaneously. The caring doctor gave lifesaving treatment to an elderly woman with "do not resuscitate" instructions.

As Meredith realized the gravity of the matter, the patient's friends, played by June Lockhart, Betty Garrett and Rae Allen, served as a tsk-tsking trio. Allen's character then whacked Meredith with a large handbag, calling her a schnook.

Pompeo broke up laughing during a couple of takes. Working with the veteran actresses was refreshing on a demanding day. "You can only hope for days like this," Pompeo says. "They're hysterical."

DOCTOR'S ORDERS

Though the medical plots exist to serve the characters, often serving as a metaphor for events in their lives, the show wants to get them right. It has featured such oddities as a teratoma, a growth that can make a man appear pregnant.

On the operating room set, director David Paymer gave instructions to Dempsey, Heigl and Walsh, whose characters were trying to save a premature baby. The actors were playful between takes — Dempsey and Walsh pantomimed patty-cake over the dummy mother — but the lifesaving scene was all business.

That seriousness grounds everything else, says co-executive producer Peter Horton. "Human beings in direct proximity of life and death — that makes it intriguing, entertaining and extremely moving."

VARIETY IS THE SPICE

"Grey's" is running ahead of a TV world slow to reflect the country's diversity. The cast includes black and Asian-American doctors; a Latina character is coming. But they are not defined by race. Cristina Yang (Sandra Oh), an Asian-American, and Preston Burke (Isaiah Washington), an African-American, have relationship issues, but they are based on personality.

"Grey's" "actually reflects a regular city, with three-dimensional minority characters," says Kevin Lockett, a viewer from Akron, Ohio.

When "Grey's" was casting, the goal was to be open to all actors without earmarking roles by race.

"Grey's" diversity "is by design and not by design at the same time," says Rhimes, the lone black woman heading a broadcast network drama series. It also features strong, complex women pursuing professional and personal goals. "It's an accurate reflection of the women I know."

RX: SEX, AND LOTS OF IT

From the hot cast — Dempsey is in People magazine's Sexiest Man Alive issue and other cast members have not-so-secret admirers — to trysts ranging from romantic to lustful, "Grey's" offers soapy va-va-va-voom, creating buzz and drawing media interest.

The yin-yang of Burke-Yang intrigues fans, as does the slow-moving mating dance of Izzie and Alex.

Last week's renewed spark between Addison and McDreamy left viewers buzzing.

"You get to see them try to work it out," Walsh says. But "that's a very small trailer to live in."