(1) Determine the accuracy of self-reported height, weight, and body mass index (BMI) calculated from those values in a population suffering from both serious mental illness (SMI) and overweight/obesity; (2) identify any associations that may predict error in self-reported measurements. Data were collected from screening appointments for two clinical trials for adult patients with SMI and overweight/obesity (BMI > 28) who gained weight while on antipsychotic medications. Both studies were conducted at the same urban community mental health center. Differences in self-reported and measured height, weight, and BMI were calculated. Analysis included age, sex, race, psychiatric diagnosis, and level of education. BMI calculated from self-reported height and weight were significantly lower (-0.47kg/m2) than measured values. Height was significantly overestimated (1.04cm), while weight was underestimated (0.055kg). Men underestimated BMI more than women (0.55 vs. 0.41kg/m2). Increasing age correlated with lower accuracy of self-reported height and BMI. No differences due to psychiatric diagnosis, race, or education were found. BMI calculated from self-reported height and weight from patients with SMI and overweight/obesity is as accurate as the self-reported measurements collected from the general population and, while measurement is best, self-reports can be used as a tool for screening for obesity.