Thorough medical evaluation of 529 patients drawn from eight program categories in California's public mental health system revealed active, important physical disease in 200 patients who had 291 diseases. Fourteen percent of the patients had diseases known to themselves but not to the mental health system, and 12% of the patients had diseases newly detected by the study team. We estimate that of the more than 300,000 patients treated in the California public mental health system in fiscal year 1983 to 1984, 45% had an active, important physical disease. The mental health system had recognized only 47% of study patients' physical diseases, including 32 of 38 diseases causing a mental disorder and 23 of 51 diseases exacerbating a mental disorder. Patients treated in public sector mental health facilities should receive careful medical evaluations.