We report ease of conversion, effect on equivalent efficacy and frequency of dosing when immediate-release carbidopa/levodopa (Sinemet, IR-CL, DuPont Pharma, Wilmington, DE, U.S.A.) is replaced with controlled-release carbidopa/levodopa (Sinemet CR, CR-CL, DuPont Pharma, Wilmington, DE, U.S.A.) in patients with Parkinson's disease (PD). One-step conversion through the application of a mathematical formula was utilized. Fifty-two patients (35 men, 17 women) with a mean ± SD age of 72 ± 8 years participated in this open-label study. All patients were taking IR-CL prior to conversion. The Unified Parkinson's Disease Rating Scale (UPDRS) was used for efficacy assessment. Pharmacokinetic studies were undertaken in five patients while they were on IR-CL, and repeated after they were switched to CR-CL. Dosage adjustment was not required either immediately after converting or during the 6-month follow-up. No significant changes occurred in efficacy scores during follow-up visits, indicating the effectiveness of the conversion. There were substantial differences in the levels of plasma DOPA, dopamine, 3-O-mcthyldopa, homovanillic acid, and dihydroxyphenylacetic acid between patients receiving IR-CL and those receiving CR-CL, but the differences did not correlate with clinical changes, suggesting that plasma levels do not reflect tissue levels at the site of action for levodopa. This study demonstrates that conversion from IR-CL to CR-CL for optimal dosing may be achieved in one step through the application of a mathematical formula with case of conversion and reduction of number of doses.