We studied the medical comorbidity among individuals with serious mental illness who were receiving community-based psychiatric treatment. A total of 200 psychiatric outpatients divided between those with schizophrenia and affective disorder diagnoses were recruited from samples receiving outpatient care at two psychiatric centers. Interviews used questions from national health surveys. Logistic regression analyses compared responses from each sample with those of matched subsets of individuals from the general population. Both patient groups had greater odds of having many medical conditions. The odds of respiratory illnesses remained elevated in the patient groups even after controlling for smoking, as did the odds of diabetes in the affective disorder group after controlling for weight. Persons with serious mental illness who are in outpatient care are more likely to have comorbid medical conditions than persons in the general population. The odds of diabetes, lung diseases, and liver problems are particularly elevated. These findings underscore the need for intensified preventive health interventions and medical services for this population.