There have been relatively few studies examining sleep in patients with obsessive-compulsive disorder (OCD) and these have produced contradictory findings. A recent retrospective study identified a possible association between OCD and a circadian rhythm sleep disorder known as delayed sleep phase syndrome (DSPS). Patients with this pattern of sleeping go to bed and get up much later than normal. They are unable to shift their sleep to an earlier time and, as a result, suffer considerable disruption to social and occupational functioning. In this study, we examined the sleep of patients with OCD prospectively. We aimed to establish the frequency of DSPS in this population and any associated clinical or demographic factors which might be implicated in its aetiology.