Nurse Judy and Drs. Turk, Reid, Kelso, Cox, and Dorian 

Physicians on duty included:

• Zach Braff as Dr. John "J.D." Michael Dorian, attending physician and staff internist (who narrates the show).
• Sarah Chalke as Dr. Elliot Reid, JD's romantic interest and later private practitioner
• Donald Faison as Dr. Christopher Duncan Turk, Surgical Attending who is JD's best friend since college and roommate until Turk marries Nurse Carla Espinosa.
• John C. McGinley as Dr. Percival "Perry" Cox, senior attending physician and the hospital's Residency Director.
• Ken Jenkins as  Dr. Robert "Bob" Kelso, Sacred Heart's Chief of Medicine. Kelso wife, Enid (unseen), a  morbidly obese paraplegic. Their son, Harrison (also unseen) is homosexual and a failure (according to his dad).
• Robert Maschio as Dr. Todd "The Todd" Quinlan, Cosmetic Surgery Fellow/Surgical Attending
• Johnny Kastl as Dr. Doug Murphy, Pathology MD
• Travis Schuldt as Dr. Keith Dudemeister, MD Resident
• Charles Chun as Dr. Wen, Surgical Attending
• Michael Hobert as Lonnie. MD Resident
• Frank Encarnacao as Dr. Mickhead
• Elizabeth Banks as Dr. Kim Briggs, Urologist
• Heather Graham as Dr. Molly Clock, Attending Psychiatrist
• Jay Kenneth Johnson as Dr. Matthews
• Bob Clendenin as Dr. Zeltzer
• Donald Sage Mackay as Doctor
• Bellamy Young as Dr. Miller
• Bob Bencomo as Colonel Doctor
• Robert E. Beckwith as Dr. Johnson
• Matt Winston as Dr. Jeffrey Steadman
• P.D. Mani as Dr. Simotas
• Julie Hudson as Dr. Ginny Gerson
• Dave Foley as Dr. Hendrick
• Ravi Patel as Dr. Patel
• Ned Bellamy as Dr. Green
• Marshall Manesh as Dr. Akbar
• Lo Ming as Dr. Kim
• Biff Yeager as Dr. Walch
• Michael J. Fox as Dr. Kevin Casey
• Rene Hall as Intern
• Chris Dougherty as Intern
• Scott Rabideau as Intern
• Artie O'Daly as Intern

Other support personnel included:

• Judy Reyes as Nurse Carla Espinosa, Head Nurse (and girlfriend/wife of Dr. Turk)
• Aloma Wright as Nurse Laverne Roberts
• Monica Allgeier as Nurse Tisdale
• Rick Schroder as Nurse Paul Flowers
• Yvonne Arias as Nurse Martinez
• Ali K. Miller as Nurse
• Janelle Giumarra as Nurse
• Lynn Meneses as Nurse
• Mona Weiss as Nurse
• Keli Daniels as Nurse
• Debbie Heimowitz as Nurse
• Amy Rilling as Foxy Nurse
• Angee Hughes as Nurse
• Lynne Maclean as Nurse
• Paige Peterson as Cute Nurse
• Derek Miller as Orderly
• Jonathan Garner as Orderly
• Derek Miller as Orderly
• Tom Schmid as Fellowship Director
• Sam Lloyd as Theodore "Ted" Buckland, Hospital Attorney
• Christa Miller as Jordan Sullivan, Sacred Heart Hospital Board Member and Perry Cox's ex-wife
• Sarah Lancaster as Lisa the Gift Shop Girl
• Neil Flynn, the "Janitor" who has a running adversarial relationship with J.D.
• Michael Coleman as another Janitor