The pharmacokinetics of gentamicin in 34 febrile neutropenic patients (40 courses) were compared with those in 40 nonneutropenic patients receiving the drug. No pharmacokinetic differences were seen in half-life, volume of distribution (liter per kilogram; total and ideal body weight), or clearance (milliliter per minute per 1.73 m2). The incidences of nephrotoxicity in the two groups were not statistically different. Because of the small number of patients with positive cultures, no relationship between initial peak serum gentamicin concentration and mortality could be determined. Mortality risk factors that were determined to be statistically important included presence of pneumonia, persistent fever in the presence of anti-infective therapy of more than 1 week duration, and peak serum creatinine of greater than 1.2 mg/dl. Initial aminoglycoside dosing in the febrile neutropenic patient should be similar to that in the nonneutropenic patient, with concentrations in serum monitored and doses adjusted accordingly.