Newly introduced dopamine agonists, such as ropinirole, may offer advantages compared to such older drugs as bromocriptine in patients with advanced Parkinson's disease (PD) with response oscillations or waning efficacy. Dose equivalence of these two drugs, however, has not been well established, which may complicate switching in clinical practice. In 23 such patients with advanced PD no longer satisfactorily responsive to prolonged bromocriptine therapy (mean dose: 18.9 +/- 6.5 mg/d), we prospectively switched the medication to ropinirole administered at three different dose-ratios (5:1, 3:1, and 2:1), increased at monthly intervals. Selegiline remained unmodified in all 17 patients receiving this medication. A dose-ratio of bromocriptine to ropinirole of close to 2:1 (1.87; mean ropinirole dose: 10.1 +/- 2.5 mg/d) was the only dose that significantly reduced mean motor Unified Parkinson's Disease Rating Scale (UPDRS) scores ( p = 0.030, analysis of variance). Individually considered, however, four patients (21%) scored worse even at this dose-ratio when compared to baseline assessment on bromocriptine. "Off" time was reduced by 57.3% in fluctuating patients, and the dyskinesia score decreased by 53.8%, although the changes were not statistically significant. Higher bromocriptine to ropinirole dose ratios (i.e., 5:1 and 3:1) resulted in "off"-time increases in half of the patients with fluctuations, and two previously stable patients developed a wearing-off effect and one other patient experienced off-time dystonia. One patient developed dose-dependent dopaminomimetic psychotic symptoms with ropinirole. In conclusion, "off"-time motor scores and possibly "off"-time duration, and severity of dyskinesias in patients with advanced PD with prolonged bromocriptine therapy may improve in a majority of cases by switching to ropinirole, provided that the latter drug is administered at a dose ratio of 2:1 compared to bromocriptine. Higher dose ratios are often ineffective or may even cause a clinical worsening of symptoms in some patients.