To determine if medical students who demonstrate unprofessional behavior in medical school are more likely to have subsequent state board disciplinary action. A case-control study was conducted of all University of California, San Francisco, School of Medicine graduates disciplined by the Medical Board of California from 1990-2000 (68). Control graduates (196) were matched by medical school graduation year and specialty choice. Predictor variables were male gender, undergraduate grade point average, Medical College Admission Test scores, medical school grades, National Board of Medical Examiner Part 1 scores, and negative excerpts describing unprofessional behavior from course evaluation forms, dean's letter of recommendation for residencies, and administrative correspondence. Negative excerpts were scored for severity (Good/Trace versus Concern/Problem/Extreme). The outcome variable was state board disciplinary action. The alumni graduated between 1943 and 1989. Ninety-five percent of the disciplinary actions were for deficiencies in professionalism. The prevalence of Concern/Problem/Extreme excerpts in the cases was 38% and 19% in controls. Logistic regression analysis showed that disciplined physicians were more likely to have Concern/Problem/Extreme excerpts in their medical school file (odds ratio, 2.15; 95% confidence interval, 1.15-4.02; p =.02). The remaining variables were not associated with disciplinary action. Problematic behavior in medical school is associated with subsequent disciplinary action by a state medical board. Professionalism is an essential competency that must be demonstrated for a student to graduate from medical school.