A high incidence of obsessions and compulsions is documented in basal ganglia disorders, especially in patients with Tourette's syndrome (TS). A comparison of patients with obsessive-compulsive disorder (OCD), TS, and Parkinson's disease (PD) revealed significantly higher total scores in both OCD and TS patients than in a healthy control group on the Maudsley obsessive-compulsive inventory (MOCI) and the Hamburg obsessive-compulsive inventory (HZI-K), two self-report measures of obsessive-compulsive symptoms. On most subscales (especially Checking, Ordering, and Counting/touching), TS patients scored higher than controls. Patients with Parkinson's disease merely scored higher on the subscale 'Ordering' of the HZI-K. Differences between OCD patients and TS patients were evident on the MOCI subscales 'Checking' and 'Slowness/Repetition' as well as on the MOCI total score and on the HZI subscales 'Cleaning' and 'Obsessive Thoughts'. On these scales, TS patients reported fewer symptoms than OCD patients. Stepwise discriminant analysis with preselected single items as variables was used to look for specific symptom patterns of OCD and TS. Seventy-eight percent of the patients could be correctly classified with respect to their diagnoses on the basis of only two items of the HZI-K. One item asks for fearful obsessive thoughts, which was found in 90% of the OCD patients; the second item represented echo phenomena, found in 56% of the TS patients. It is concluded that considering specific patterns of obsessive-compulsive psychopathology may contribute to a more reliable differential diagnosis in OCD and TS and help to avoid misdiagnosis of OCD in TS patients.