Explored the possibility of a biological basis for obsessive compulsive disorder by examining topographical EEG recordings in 30 16-45 yr old patients diagnosed with primary obsessional disorder. Results show that 90% of the patients showed EEG abnormalities. 15 patients showed left hemispheric changes; 11 of these had focal hemispheric dysfunction, 4 in the temporal region and 7 in the posterior quadrant. These results point to the prevalence of left hemisphere dysfunction and abnormal cognitive processing. The authors discuss possible subtypes of the biological substrate of obsessive compulsive disorder.