Vitamin B12 and folate serum levels were studied in 30 patients with obsessive compulsive disorder (OCD), and in two control groups comprised of 30 chronic schizophrenics and 30 normal healthy subjects. Six patients (20%) of the OCD group had abnormal low levels of vitamin B12. This prevalence was significantly higher than that of the control groups. No clinical neurological or haematological abnormalities accompanied the reduced vitamin B12 levels. Possible implication of this finding for the pathophysiology of OCD in a subgroup of patients and the possibility that the B12 deficiency could be the consequence rather than the cause of OCD are suggested.