The aim of the study was to predict dosing with gentamicin (Ge) of which the effect is dependent on plasma concentrations (Cpl) more than on dosage, to achieve the target range of steady state through Cpl: Ctrough,3 (0.5–2.0 mg/l) and peak Cpeak,4 (5.0–10.0 mg/l), i.e. 0.5 h before the fourth and one h after the start of the fourth dose. Cpeak,4 determines bactericidal effect, Ctrough,3 predicts neurotoxicity and nephrotoxicity.