Objective: To examine the co-existence of obsessive-compulsive disorder (OC
D) with schizophrenia in terms of epidemiology and overlapping biologic sub
strates. Methods: Review of the relevant literature. Results: There appears
to be a significant prevalence of OCD in schizophrenia - higher than what
would be expected on the basis of calculated comorbidity figures. There is
significant overlap in the proposed functional circuits of OCD and schizoph
renia, which may lead to co-expression of symptoms. Although there is overl
ap in neurotransmitter dysfunction, the interactions are complex, especiall
y in regard to the serotonin and dopamine systems. Conclusion: The expressi
on of OCD in schizophrenia is complex but very intriguing. Theoretical hypo
theses of the pathology of the 2 disorders now need to be tested in larger
controlled trials.