162 consecutive gentamicin courses have been evaluated retrospectively with respect to nephrotoxicity of gentamicin (GM). Of these, 120 courses were administered in 106 patients for more than 2 days and under adequate control of plasma creatinine (PCr). In 62 of these 120 courses, PCr concentrations increased. In 17 courses (14%), GM therapy was found to be the only demonstrable etiology to the rise in PCr. The 17 courses with GM-induced reduction in kidney function were characterized by a prolonged duration of treatment, a high total dose of GM and a somewhat higher level of serum GM than the 58 courses of GM treatment in which PCr remained unchanged. No significant differences were found with regard to age, average daily dose of GM, average daily dose per kg and average daily dose in proportion to average diuresis. Additional administration of other nephrotoxic drugs did not increase the incidence of GM-induced nephropathy. When GM was the only demonstrable cause of nephropathy, the elevation in PCr concentrations were generally mild and transient, while the nephropathy when other factors were involved more often became severe and occasionally irreversible.