Pergolide was administered to 56 patients with advanced Parkinson disease who were no longer satisfactorily responding to levodopa. The group included 45 patients with on-off phenomena. Pergolide, when combined with levodopa, resulted in a 44% decrease in disability as assessed in the on period, a 15% decrease in disability as assessed in the off period, and a 148% increase in the number of hours in which patients were on (from 4.6 +/- 0.3 hours to 11.4 +/- 0.6 hours). All these changes were significant at 1%. Forty-one of the 56 patients (59%) improved when pergolide was added to levodopa. Mean dose of pergolide was 2.5 mg (range, 0.2 to 10.0 mg). Mean duration of the study was 13 months (range, 1 day to 34 months). Maximum improvement occurred within 2 months and began to decline, usually after 6 months. The major adverse effects necessitating discontinuing pergolide were the occurrence of an organic confusional syndrome (six patients), increased dyskinesias (four patients), and cardiovascular abnormalities (three patients). Nine patients discontinued pergolide because of a lack of effect or declining effect.