We investigated the prevalence of dyskinesias and motor fluctuations, and the factors determining their occurrence, in a community-based population of patients with Parkinson's disease. Among 124 patients with Parkinson's disease, 87 (70%) had received a levodopa preparation. Among these 87 patients, 28% were experiencing treatment-induced dyskinesias and 40% response fluctuations. The prevalence of motor fluctuations was best predicted by disease duration and dose of levodopa, whereas dyskinesias could be best predicted by duration of treatment. Patients with a shorter time from symptom onset to initiation of levodopa and younger patients had developed motor complications earlier, and patients who had started treatment with a dopamine agonist had developed these treatment complications later. Although a satisfactory response to medication was associated with higher rates of motor complications, poor or moderate response was associated with lower quality of life in patients with a disease duration of </=5 years or >/=10 years. We conclude that motor fluctuations are most strongly related to disease duration and dose of levodopa, and dyskinesias to duration of levodopa treatment. However, poorer quality of life associated with inadequate dosage of levodopa may be the price for a low rate of motor complications in patients with Parkinson's disease.