We conducted a 20-week nonblind study to evaluate the efficacy of piribedil in 30 patients with idiopathic Parkinson's disease (PD). Prior to the study 17 of these patients were under L-Dopa treatment alone or in combination with anticholinergics and/or amantadine, while 13 patients who had never taken L-Dopa were treated only with anticholinergics and/or amantadine, or were without any medication. Piribedil (in the retard form) was administered orally at a gradually increasing dose up to 200 mg daily, while previous antiparkinsonian medication remained unchanged. Twenty-five patients showed statistically significant improvement. Among the cardinal symptoms of parkinsonism, tremor responded the best. Depression also appeared to respond favorably. Our results indicate that piribedil may be a useful adjuvant in the treatment of PD.