The predictive performance of a dosage calculation method for the optimization of tobramycin therapy was studied retrospectively in 29 patients with cystic fibrosis. The dosage calculation method was based on a linear one-compartment open model. It used peak and trough serum concentrations of the aminoglycoside. Bias in the peak concentration was 0.20 mg/L and precision was 1.2 mg/L. Bias in the trough concentration was -0.06 mg/L and precision was 0.33 mg/L. The results were clinically satisfactory. Comparison with previously published results of the predictive performance of other dosage calculation methods showed that the method studied was at least as good.