BACKGROUND: Comorbid mood disorders affect the prognosis of obsessive-compulsive disorder (OCD) negatively. Affective temperaments are assumed to be subsyndromal symptoms and precursors of mood disorders, but its effects on OCD outcome still remain unclear. There is a body of evidence, which supports the association between circadian rhythm disturbances and mood disorders in the literature. In contrast, there is limited data concerning the effects of chronobiological differences among the patients with OCD and OCD comorbid mood disorders. The main objective of this present study was to examine the clinical effects of affective temperaments and chronotype differences in patients with OCD.