The disposition of zidovudine (ZDV) was examined during chronic oral dosing (300 mg every 4 h while awake) for 12 weeks in eight asymptomatic patients with hemophilia who were infected with the human immunodeficiency virus. Pharmacokinetic studies were conducted at the initiation of drug administration and after 6 and 12 weeks. Baseline liver function tests indicated normal values for bilirubin, albumin, and prothrombin time, while hepatic enzyme levels ranged from one to three times the normal levels. Initially, the mean peak ZDV concentration in plasma was 2,052 ng/ml with a range of 1,033 to 3,907 ng/ml, while during chronic dosing the peaks were 1,619 +/- 1,062 ng/ml and 1,711 +/- 786 ng/ml at weeks 6 and 12, respectively. ZDV concentrations at 4 h declined to 77 +/- 53 ng/ml, 110 +/- 43 ng/ml, and 101 +/- 49 ng/ml at weeks 1, 6, and 12, respectively. Initially, the plasma concentration-versus-time decay in three patients was linear, with a mean half-life (t1/2) of 1.3 +/- 0.5 h, while five patients had detectable concentrations in plasma after 4 h with an apparent delayed terminal-phase t1/2 of 4.8 +/- 2.8 h. At week 6 the prolonged elimination pattern was noted in all patients (terminal t1/2 = 4.1 +/- 2.0 h). No correlation between hepatic enzyme levels and t1/2 was noted. These findings suggest that ZDV may display a prolonged elimination phase during multiple dosing. Further studies utilizing a more sensitive assay may help to further define this later phase of ZDV elimination.