Data from 42 patients were analyzed to determine the predictability of the peak serum level and the t 1/2 of gentamicin on the bases of age, sex, body weight, serum concentration of creatinine, and blood hematocrit. Renal function was normal in 21 patients and impaired in 21. The most striking finding was the relatively poor predictability of t 1/2 of gentamicin from serum concentration of creatinine. The overall correlation coefficient was 0.749 (P < 0.(01) in contrast to values of >0.9 reported by others. There was a significant correlation of the t 1/2 of gentamicin with the reciprocal of hematocrit (r = 0.647, P < 0.001). Linear regression equations taking account of sex, serum creatinine concentration, and reciprocal of hematocrit provided a somewhat higher correlation coefficient (0.821) with the t 1/2 of gentamicin than did the equation including serum creatinine concentration alone but were still not fully satisfactory. Thus the pharmacokinetics of gentamicin may not be adequately predictable from standardized equations or nomograms.