Pharmacokinetic parameters of zidovudine (ZDV) were not altered in 16 patients receiving concomitant therapy with ZDV and trimethoprim-sulfamethoxazole by oral administration. ZDV areas under the concentration-time curves were (means +/- standard deviations) 1.80 +/- 0.70 and 1.69 +/- 0.64 mu g . h/ml in the absence and presence of trimethoprim-sulfamethoxazole, respectively. ZDV clearances were 1.57 +/- 0.61 and 1.74 +/- 0.66 liters/h/kg, respectively.