We report on the clinical efficacy of a slow-release formulation of bromocriptine studied in a multi-center, double-blind trial using standard bromocriptine as the control. We randomly allocated enrolled patients (N = 243) to either the slow-release or normal bromocriptine group. Sixty of them were de novo patients. The maintenance dose of slow-release bromocriptine was 14.2 +/- 0.7 mg/d and that of standard bromocriptine 13.5 +/- 0.7 mg/d (mean +/- SE). The slow-release formulation was taken twice and the standard three times a day. Forty-one percent of the patients treated with the slow-release bromocriptine and 32% of the patients treated with the standard bromocriptine showed moderate or marked improvement in the global improvement rating. There were no serious side effects, and the frequency of vomiting and epigastric discomfort was lower in the patients treated with the slow-release bromocriptine. Clinical efficacies for tremor, rigidity, akinesia, and gait disturbance were comparable between the two drugs tested. The slow-release bromocriptine seems to be a valuable drug for the treatment of Parkinson's disease with less severe side effects than regular bromocriptine.