A Doctor's Diary is told through the eyes of Dr. Dan Norris (John Trent), resident physician in a private hospital. In his search for a cure for spinal meningitis, Norris recklessly runs roughshod over the feelings of his colleagues. The doctor's older, wiser supervisor, Dr. Clem Driscoll (George Bancroft), tries to curb Norris' impatience, pointing out that nothing takes place overnight. Angrily, Norris accuses Driscoll of malpractice and is forced to resign from the clinic -- just when a meningitis epidemic breaks out.
| Actor | Role |
|---|---|
 George BancroftDr. Clem Driscoll | Dr. Clem Driscoll |
 Helen BurgessRuth Hanlon | Ruth Hanlon |
 John TrentDr. Dan Norris | Dr. Dan Norris |
 Ruth ColemanCatherine Stanwood | Catherine Stanwood |
 Ronald SinclairMichael Fielding (as Ra Hould) | Michael Fielding (as Ra Hould) |
 Molly LamontMrs. Fielding | Mrs. Fielding |
 Sidney BlackmerDr. Anson Ludlow | Dr. Anson Ludlow |
| CCharles WaldronDr. Ellery Stanwood (as Charles D. Waldron) | Dr. Ellery Stanwood (as Charles D. Waldron) |
 Frank PugliaLouie | Louie |
 Milburn StoneFred Clark | Fred Clark |
 Sue CarolMrs. Mason | Mrs. Mason |
 Mickey KuhnBoy in Hospital (uncredited) | Boy in Hospital (uncredited) |