Rasagiline mesylate (TVP-1012) is a potent, selective, non-reversible MAO-B inhibitor, without the tyramine-potentiating effect and with neuroprotective activities. The benefit of rasagiline as monotherapy in patients with early Parkinson's disease (PD) has already been reported. To evaluate the safety, tolerability, and clinical effect of rasagiline as adjunctive therapy to levodopa, a multicenter, double-blind, randomized, placebo-controlled, parallel-group study (0.5, 1, and 2 mg/d) was conducted for 12 weeks in 70 patients with PD (mean age, 57.4 y; mean disease duration, 5.7 y; 32 patients had motor fluctuations). A beneficial clinical effect was observed in fluctuating patients treated with rasagiline (all doses), expressed as a decrease in total Unified Parkinson's Disease Rating Scale (UPDRS) score (23.0% vs 8.5% in the placebo group). The treatment effect was still evident 6 weeks after drug discontinuation (in all doses). The safety and tolerability of rasagiline were good. Adverse events were no different than those of patients taking placebo. Almost complete platelet MAO-B inhibition was obtained at all rasagiline doses. This study has demonstrated that rasagiline (up to 2 mg/day) has a good safety profile and a beneficial clinical effect in fluctuating patients with PD when given as an add-on to chronic levodopa therapy.