Background: : Psychiatric disorders are associated with overweight/obesity. Obsessive-compulsive disorder (OCD) may be an exception, as anecdotal evidence suggests lower BMI in OCD. Additionally, depression is associated with elevated BMI, but effects of comorbid secondary depression are unknown. The aim of the present study was to assess BMI and risk for overweight/obesity in OCD and to assess the effect of comorbid depression on BMI. Methods: : BMI, demographics, and clinical status were assessed in large samples of individuals with OCD, anxiety disorders, depressive disorders, comorbid anxiety/depressive disorders, and non-clinical controls (NCC). Results: : Although no initial differences were found between the samples on BMI, the non-depressed OCD subsample had significantly lower BMI and risk for overweight/obesity compared to all other clinical samples. NCC were nearly twice as likely to be overweight compared to non-depressed OCD. Limitations: : Eating disorders were excluded in the OCD sample, but BMI < 17 was used as an exclusion criterion in the clinical control groups in lieu of screening for Anorexia. Group differences on demographics were controlled for. Recruitment methodology differed between samples. Conclusions: : OCD is associated with significantly lower rates of obesity and overweight, but this relationship was not found when comorbid depression was present. This suggests that the purer the phenotype of OCD, the more substantial protective factor against overweight/obesity emerges compared to other clinical samples and NCC. An OCD-specific reward/anhedonia model, previously offered to elucidate lower smoking rates in OCD, may account for lower BMI in OCD. These results warrant careful clinical attention to the negative impact of comorbid depression on OCD that spans from increasing risk for obesity and cigarette smoking, to hindering treatment response.