Serum kinetics of trimethoprim and sulfamethoxazole were studied in 23 patients during oral and i.v. treatment of Pneumocystis carinii pneumonia. Daily doses of 15-22 mg/kg trimethoprim and 75-110 mg/kg sulfamethoxazole were given every 6 h. Despite administration of a loading dose of twice the regular dose, serum trough concentrations continuously rose from 12 h to 96 h by 63% for trimethoprim and 102% for sulfamethoxazole. After 4-6 days mean trough concentrations of trimethoprim and sulfamethoxazole were 7.7 +/- 3.0 and 198 +/- 74 mg/l, with individual values of < 4.6 and < 103 mg/l in two patients and > 11.4 and > 307 mg/l in two others. Patients treated orally or i.v. had similar serum levels. However, large interindividual variability was observed despite weight-specific dosing. Administration of a loading dose did not prevent accumulation of serum levels of trimethoprim and sulfamethoxazole over several days of treatment.