In this Danish-Norwegian randomized double-blind parallel-group multicentre study, we compared the therapeutic response of slow-release Madopar HBS to standard Madopar in 134 de novo patients with idiopathic Parkinson's disease during a 5-year period. The drugs were dosed according to the individual need of the patients. The Webster, NUDS, UPDRS and Hoehn & Yahr scales were used for evaluation of symptoms. Addition of a morning dose of standard Madopar 62.5 mg was allowed after 6 months. Bromocriptine could be administered but not Selegiline. Sixty-five patients got Madopar HBS and 69 standard Madopar. Surprisingly, no differences were found as to the mean daily levodopa dose, the mean number of daily doses or the use of the doses of bromocriptine. Unexpectedly, we found a trend towards a more frequent use of a morning dose of standard Madopar in the group treated with the standard formulation. No differences were observed in the occurrence of motor fluctuations or dyskinesia, the incidence of which was relatively low. Sustained-release Madopar (HBS) thus proved to be as effective as standard Madopar in the long-term treatment of de novo parkinsonian patients, but the drug showed no advantage in postponing or reducing the long-term levodopa treatment problems.