Although physical inactivity is a leading cause of death and the Surgeon General recommends regular moderate physical activity, many Americans are inactive. Because of their increased burden of obesity and diabetes, people with severe mental illness (SMI) especially may benefit from physical activity, yet little is known about the prevalence and types of physical activity in people with SMI. We surveyed outpatients with schizophrenia and affective disorders at two psychiatric centers in Maryland and compared physical activity patterns to an age-gender-race-matched national sample (National Health and Nutrition Examination Survey III) of the general population. We found that people with SMI are overall less physically active than the general population, although the proportion with recommended physical activity levels was equal. The participants with SMI were more likely to walk as their sole form of physical activity. Within the SMI group, those without regular social contact and women had higher odds of being inactive.