We studied the risk factors that influence the occurrence of response fluctuations and dyskinesias in 194 patients with Parkinson's disease (PD). These factors included age at onset, disease severity at levodopa initiation, duration of illness prior to levodopa therapy and the daily dose of levodopa at the end of study. The first three were fixed covariates and the last one was time-dependent. Patients with age onset before 50 years showed a higher risk of developing response fluctuations (RF) and dyskinesias in univariate analysis. We estimated the combined effects of these covariates through multivariate analysis and found that the age at onset was the sole factor related to the incidence of both complications. Additionally, it was the only variable that could enter the predictive model of stepwise regression. Our observation suggested that age at onset determined the occurrence of RF and dyskinesias. Patients with younger age at onset had higher risk of developing these adverse effects.