In five patients with parkinsonism, the optimal dosage of a controlled-release levodopa/carbidopa preparation (CR-3) was three times higher than the dosage of Sinemet and produced higher plasma levodopa concentrations, but did not reduce the fluctuations in plasma levodopa or clinical response. Plasma levodopa concentrations were higher and clinical responses better before the first dose of the day with CR-3. CR-3 treatment benefited two patients, reducing the severity of off periods and off dystonia. Two patients were worse on CR-3 despite higher plasma levodopa levels than those adequate for clinical response to Sinemet or levodopa infusions. CR-3 could benefit a few severely affected patients, but it is necessary to understand the factors that affect absorption of levodopa from sustained-release preparations, as well as the consequences of prolonged elevation of plasma levodopa levels.