The objective of this study is to conduct a dose-finding study of sarizotan in Parkinson's disease (PD) patients with dyskinesia to identify a safe dose and to identify a sensitive dyskinesia rating measure. Sarizotan is a novel compound with full 5-HT(1A) agonist properties and additional high affinity for D(3) and D(4) receptors. An open label study documented improvements in PD patients with levodopa-induced dyskinesia. There is no precedent for study designs or outcome measures in pivotal trials of antidyskinesia therapies. The approach used here was a multicenter, randomized, placebo-controlled, double-blind, parallel study. Included were PD patients optimized to levodopa and dopaminergic drugs with moderately disabling dyskinesias present greater than or equal to 25% of the waking day. Interventions included sarizotan 2, 4, or 10 mg/day or matching placebo, given in two doses. There were two outcome measures: the primary measure was change from baseline in diary-based on time without dyskinesia; the secondary measures were change from baseline in scores on the Abnormal Involuntary Movement Scale (AIMS), the composite score of Unified Parkinson's Disease Rating Scale (UPDRS) Items 32+33 (dyskinesia duration and disability) and total UPDRS. A total of 398 subjects were randomized, with 381 included in the intention-to-treat population. No significant changes occurred on sarizotan compared to placebo on any diary-based measure of dyskinesia or the AIMS score. The composite score of UPDRS Items 32+33 was significantly improved with 2 mg/day sarizotan, with a trend at 10 mg/day. Adverse events were not significantly different in sarizotan- and placebo-treated patients, but off time significantly increased with sarizotan 10 mg/day. Sarizotan 2 mg/day is a safe agent in PD patients with dyskinesia. To test its role in abating dyskinesia, future studies should focus on this dose and will use the composite score of UPDRS Items 32+33 as the primary outcome.