The safety of entacapone combined with levodopa and a dopadecarboxylase (DDC) inhibitor was tested in a 12-month double-blind study of 326 patients with idiopathic Parkinson’s disease (PD). The study population represented ‘typical’ PD outpatients, including patients with varying disease severity and with various concomitant medications. Two-thirds of the patients were randomized to receive 200 mg of entacapone with each of 2–10 daily levodopa doses, and one-third to receive placebo. All entacapone patients were included in the safety evaluation of adverse events (AEs), vital signs, ECG, and laboratory parameters. Entacapone was well tolerated with a discontinuation rate due to AEs of 14% compared with 11% with placebo (NS). As expected, due to dopaminergic enhancement, dyskinesia was more frequent as an AE with entacapone than with placebo. Dryness of mouth, urine discoloration and diarrhoea were more frequent non-dopaminergic AEs with entacapone than with placebo. Entacapone had no adverse effects on hepatic enzyme activity, ECG or haemodynamic parameters, and there was no evidence of any toxicity. As an indication of levodopa enhancement with entacapone, patients taking 5–10 doses of levodopa, most likely representing predominantly fluctuating patients, showed a significant decrease in their mean daily levodopa dose of 94 mg in the entacapone group compared with a decrease of 39 mg in the placebo group (P < 0.01). The interval between the first two morning doses of levodopa increased by 17% with entacapone, whereas with placebo no extension was observed (P < 0.05). Despite levodopa dose reduction, efficacy of entacapone was maintained. As further evidence of efficacy, Parkinsonian symptoms markedly worsened in all patients after withdrawal of entacapone. We conclude that entacapone is safe in optimizing levodopa in long-term treatment of idiopathic Parkinson’s disease. Monitoring of liver or other safety parameters during entacapone treatment is not required.