Pharmacokinetic studies of intravenously administered trimethoprim-sulfamethoxazole (TMP-SMX) were conducted in 11 patients with cancer while they received therapy with this drug combination for infection. Each patient received 160 mg of TMP and 800 mg of SMX every 8 h. The highest plasma concentrations of both agents were attained at the end of a 1-h infusion period, and the levels were maintained above 38 mug of free SMX and 2 mug of TMP per ml for 2 to 4 h on day 1. On day 4, these concentrations were exceeded at all time intervals of blood sampling. High concentrations of TMP and free SMX were recovered in the urine during the 8-h period. The plasma half-lives of TMP and free SMX, as determined during the first 8-h period, were 7.6 and 8.6 h, respectively. Compared with SMX, TMP had an approximately 2.5 times higher volume of distribution. This drug combination was well tolerated by the patients and unaccompanied by drug-related toxicity.