Obsessive-compulsive disorder (OCD) is a common psychiatric illness. Although the aetiology of OCD is still unknown, the family-genetic data show that familial forms of OCD may be associated with a specific genetic susceptibility. Recent investigations have associated development of OCD with infectious illness. Toxoplasmic encephalitis (TE) is a common presentation of Toxoplasma gondii infection of the central nervous system (CNS). The most commonly affected CNS region in TE is the cerebral hemisphere, followed by the basal ganglia, cerebellum and brain stem. The basal ganglia has been implicated in the development of OCD. Therefore, in this study, it was aimed to investigate a possible association between Toxoplasma infection and OCD. We selected 42 patients with OCD and 100 healthy volunteers, and investigated the sero-positivity rate for anti-Toxoplasma IgG antibodies by Enzyme Linked Immunosorbent Assay (ELISA). The sero-positivity rate for anti-T. gondii IgG antibodies among OCD patients (47.62%) was found to be significantly higher than the rate in healthy volunteers (19%). This is the first report to examine a potential association between Toxoplasma infection and OCD. The main finding of the present study is an increased level of IgG antibodies to T. gondii in OCD patients when compared with the level in healthy controls. There might be a causal relationship between chronic toxoplasmosis and the aetiology of OCD. (C) 2010 Elsevier Ireland Ltd. All rights reserved.