Several nomograms and computer programs are available to aid in aminoglycoside dosing. 2. Due to the variability in the relationship between dosage and serum drug levels, monitoring through the acquisition of serum drug levels is mandatory. 3. All clinical data, including serum drug levels, are subject to errors. 4. The program we have evaluated, OPT, calculates the most likely set of pharmacokinetic parameter estimates for individual patients by applying Bayes' theorem and the principle of Maximum Likelihood Estimation. Through a feedback process all available data are used, taking possible errors into account. 5. Our study shows that OPT is able to predict serum aminoglycoside levels accurately in the routine clinical setting. It may thus contribute to the quality of aminoglycoside therapy.