The derived pharmacokinetic variable estimates from a Bayesian aminoglycoside dosing program were compared with those from the Sawchuk-Zaske method to determine which variable estimates were the most accurate in fitting the test dose and in predicting subsequent peak and trough serum concentrations. Data on 17 patients with moderately impaired but stable renal function were analyzed. All patients received gentamicin sulfate for treatment of their infections. To determine the individualized variables using the Bayesian program, demographic data, dosing history, and one (midpoint), two (peak and trough), or four serum drug concentrations were entered into the program. The Sawchuk-Zaske method used three serum concentrations determined following a first dose or four concentrations before and after a subsequent dose to derive individualized pharmacokinetic variables. The estimates of pharmacokinetic variables determined using the Bayesian method with one, two, or four serum concentrations did not differ significantly from those obtained using all the available serum concentrations with the Sawchuk-Zaske method. Although the actual numeric differences of prediction, absolute, and squared errors for fitting the test dose were minimal, significant differences were seen. All methods were similar in predicting serum concentrations from continued dosing. For the prediction error from continued dosing, a slight but significant difference was observed with the Bayesian method using one serum concentration when compared with the other methods. The Bayesian method using one, two, or four serum gentamicin concentrations individualized pharmacokinetic variables as well as the Sawchuk-Zaske method.