OBJECTIVE: Assessment of cognitive functions among obsessive-compulsive disorder (OCD) patients would help in understanding the neurobiology and brain areas involved in that disorder. The objective of this work was to study the cognitive dysfunction in OCD patients and to identify its correlation with both the clinical picture and the severity of the disorder. METHOD: Neuropsychological and electrophysiological event-related potentials were tested in 30 OCD patients and compared with 30 normal volunteers of a matched gender, age and education. RESULTS: Results showed a defective visuospatial recognition, which worsens with chronicity, deteriorated set-shifting abilities, overfocused attention to irrelevant stimuli and delayed selective attention to relevant tasks. Mild cases showed better selective attention than severe cases. Obsessive cases had a defective visual memory, while compulsive cases had delayed perception of task relevant stimuli. Mixed cases showed disturbed information-processing both early and late. CONCLUSION: OCD patients have a characteristic pattern of cognitive dysfunction that differs among patients of varied severity, chronicity and symptom type. We suggest a striatofrontoparietal neural pathophysiology. OCD seems to be a heterogeneous disorder, both clinically and pathophysiologically.