A single-blind, crossover study was carried out to compare the efficacy and safety of pergolide against that of bromocriptine in 57 patients with Parkinson's disease who showed a declining response to levodopa therapy. Patients were randomly assigned to receive either bromocriptine followed by pergolide, or pergolide followed by bromocriptine. Both drugs were administered for 12 weeks. Patients were assessed by a clinician blinded to treatment assignment using the New York University Parkinson's Disease Scale. The average daily dose of pergolide was 2.3 +/- 0.8 mg and of bromocriptine 24.2 +/- 8.4 mg. Addition of pergolide or bromocriptine resulted in a significant improvement in total scores when compared with the previous treatment of levodopa alone (pergolide, p = 0.0001; bromocriptine, p = 0.0005). Pergolide was more effective than bromocriptine in daily living scores (p = 0.02) and motor scores (p = 0.038). No differences in the incidence of dyskinesias, dystonias, or psychosis were observed between groups. Fewer adverse events were recorded in the pergolide group, and most patients and physicians preferred pergolide to bromocriptine. Pergolide as adjunctive therapy to levodopa was more effective than bromocriptine in this short-term trial.