To identify putative risk factors for levodopa-induced dyskinesias we studied the effect of several clinical variables on the occurrence of dyskinesias in a series of 168 consecutive patients with Parkinson's disease treated for at least 6 months with levodopa. Of these, 108 (64%) developed dyskinesias after a mean duration of levodopa treatment of 51.4 +/- 43.3 months. Patients tended to suffer dyskinesias on the side of the body first affected by Parkinson's disease. The overall probability of developing dyskinesias increased with levodopa treatment duration, about 10% per year during the first 7 years. Univariate and multivariate logistic regression analysis identified the age at onset of Parkinson's disease (OR 0.923; 95% CI 0.883-0.964) and the initial levodopa dose (mean dose of the first 6 months of treatment; OR 1.004; 95% CI 1.002-1.006) as the main independent predictors. Survival curves showed that onset of Parkinson's disease at age 50 years or before (logrank, P < 0.05) and initial levodopa treatment with more than 600 mg/day (logrank, P < 0.05) were associated with a higher risk for the appearance of dyskinesias.