A pharmacokinetic analysis program (MULTI-Win) based on the nonlinear least squares method available in Microsoft Windows XP was developed for use in clinical practice. Using the free software, Discoversoft Active Basic, MULTI-Win was constructed by altering the MULTI program written in BASIC language included in the Windows XP operating system. This program supports clinically essential pharmacokinetic equations, which can be altered by the user, estimates drug pharmacokinetic parameters and simultaneously fits curves to observed time courses. It is also easy to operate the program on microcomputers running on Windows XP. When MULTI-Win was executed for the time course of propofol in a threecompartment model with continuous infusion, a few minutes was required to achieve accuracy and stability in the calculation.Our findings indicated that MULTI-Win would be useful in therapeutic drug monitoring for achieving a quick response in dosage adjustments for individual patients, and be helpful overall in the daily work of the clinical pharmacist.