We carried out a study of the prevalence and impact of co-existing bipolar affective disorder on patients with OCD, and the effect on their management within a community psychiatric team. Patients with this co-morbidity appeared to be challenging to treat. We found that 16% of patients who visited psychiatric outpatients with a diagnosis of OCD had co-existing bipolar affective disorder. Of these the majority had bipolar affective disorder II (67%). Compared to the control group the patients with co-morbid bipolar affective disorder had a greater number of hospital admissions,required a greater number of outpatients appointments, were more likely to have been allocated a care coordinator and to have received psychological input.