The efficacy of Sinemet CR4 (50/200) was compared to standard Sinemet (25/100) in an open label crossover study in 22 patients with Parkinson's disease. All patients experienced end of dose failure and 11 had dyskinesia. Unified Parkinson's disease, Hoehn and Yahr, Schwab and England scores, number of hours on per day, number of hours of dyskinesia per day, daily dose of levodopa, and number of doses per day were monitored at the end of each treatment period and the results compared. The only significant difference in these parameters between the CR4 and standard Sinemet treatment periods in the entire group was a decrease in hours of dyskinesia per day. Two subgroups of CR4 responders were specifically examined. The first subgroup was characterised by a significant increase in on time per day with CR4 therapy. These patients had an older age of onset of Parkinson's disease and a shorter duration of disease and fluctuations than the rest of the patients. The second subgroup was characterised by the presence of dyskinesia with standard Sinemet therapy and a significant decrease in hours of dyskinesia per day with CR4 therapy. Both subgroups required a significantly higher daily dose of levodopa while on CR4. It is concluded that CR4 may be useful in increasing hours on per day in subgroups of Parkinson's disease patients who have less severe fluctuations. It may also be useful in decreasing the number of hours of dyskinesia per day.