Individuals with severe and persistent mental illness (SPMI) have a preponderance of weight problems, possibly even greater than the obesity epidemic in the general population. Although atypical antipsychotics cause weight gain, their contribution to obesity has not been characterized in a community setting where individuals may take multiple psychotropics associated with weight gain. Using survey information including measured height and weight from a random sample of Maryland Medicaid recipients with SPMI, we compared obesity prevalence to the National Health and Nutrition Examination Survey (NHANES III) sample and a Maryland sample (Behavioral Risk Factor Surveillance System) of the general population adjusted to SPMI demographic characteristics. We investigated correlates of obesity in the SPMI sample. The results indicate that both men and especially women with SPMI had a higher prevalence of obesity than the general population; this portends substantial health implications. A fourfold association between atypical antipsychotics and prevalent obesity was found in men but not in women; further work should clarify mechanisms of obesity in the SPMI.