A one-compartment, open-linear, pharmacokinetic model for gentamicin dosing has been developed at the Maryland Institute for Emergency Medical Service Systems (MIEMSS). The model was used to predict both the gentamicin dose required to achieve desired peak and trough serum concentrations and the peak and trough serum concentrations that would result from administering empirically chosen doses. This model was tested in 31 patients, aged 15 to 82 years (mean 39.3 +/- 17.7 years), whose creatinine clearance (CCI) ranged from 12 ml/min to 197 ml/min (mean 106.9 +/- 53.1 ml/min). The predictions of the dosage model were compared with the measured peak and trough serum concentrations in these patients. The predicted peak serum levels correlated highly with the measured peak serum levels (r 0.97). The mean difference (+/- SD) between the predicted and measured peak levels was 0.28 +/- 0.22 mu g/ml. The predicted trough serum levels correlated well with the measured trough serum levels (r 0.91). The mean difference between the predicted and measured trough levels was -0.03 +/- 0.18 mu g/ml. This approach makes it possible for bactericidal levels of gentamicin to be maintained in patients with wide variations in stable renal function. Frequent serum gentamicin determinations are unnecessary. Requiring only an inexpensive calculator, the method has proved to be economical as well as clinically useful.