We administered cabergoline, a potent, once-a-day dopamine against, to 61 patients with advanced Parkinson's disease (PD) and response fluctuations--"wearing-off" and "on-off" phenomena. The patients were on stable doses of levodopa/carbidopa. During the first 5 weeks, patients were randomized to allow equal numbers to end titration at each of five daily doses of cabergoline from 0.5 to 2.5 mg. We evaluated the patients using the Unified Parkinson's Disease Rating Scale (UPDRS) and diaries of motor performance. This part of the study was double-blinded. After 5 weeks, the mean Activities of Daily Living (ADL) score on the UPDRS decreased by 22% (p < 0.0001), the mean Motor Score in the "off" period decreased by 14% (p < 0.0001), and the mean Motor Score in the "on" period decreased by 22% (p < 0.0001). The mean percent "off" time decreased by 9.0%. Twenty-three patients (38%) achieved at least a 25% improvement in the combined ADL and motor examination of the UPDRS. Four patients dropped out because of adverse effects. We treated 37 patients, including some patients who had experienced a transient 25% improvement, for an additional 8 weeks in an open manner until they achieved a 25% improvement or reached a maximum of 5 mg/d. The other patients were continued in a double-blind manner on the dose of cabergoline they had achieved at the end of week 5. At the end of 13 weeks, the group of 37 patients achieved additional significant improvements in mean ADL and mean motor scores in the "on" and "off" periods. The percent time "off" decreased by 31%.(ABSTRACT TRUNCATED AT 250 WORDS)