One hundred ninety-three asymptomatic patients with hereditary coagulation disorders and human immunodeficiency virus (HIV) infection were studied in a controlled trial of zidovudine (ZDV) versus a placebo (with an average of 9.7 months on study). Pretreatment characteristics were well balanced between the placebo and drug-treated groups, including CD4 distributions, types of clotting disorders, transaminase abnormalities, and use of various hemostatic agents. At the time of analysis, 161 patients either were still receiving treatment or had previously reached an endpoint of disease progression while receiving treatment. Twenty-five patients withdrew voluntarily. The toxic effects noted included granulocytopenia and anemia, especially in older patients, and subjective symptoms of asthenia, malaise, and nausea, consistent with the known consequences of treatment with 300 mg ZDV five times daily. There was a trend toward more diagnoses of acquired immunodeficiency syndrome (AIDS), advanced or early AIDS-related complex (ARC), single ARC symptoms, or death in placebo recipients as compared with those receiving ZDV (22 v 13). Because older patients with hemophilia have more rapid disease progression, the same efficacy analysis was performed in the 89 patients aged more than 30 years who were receiving treatment. In this subgroup, there was a similar trend (11 v 6). With regard to the most advanced problems of the infection among the older patients, there were five patients who were newly diagnosed with AIDS or died in the placebo group versus none in the ZDV group (P = .02) among the older patients. The pretreatment distribution of CD4 counts for the placebo and ZDV groups were similar, but patients aged more than 30 years had significantly (P less than .049) fewer CD4 cells than patients aged less than 30 years. A beneficial ZDV effect is also supported by a trend toward higher CD4 counts (a 48-cell increase in the ZDV group at 24 weeks as compared with a four-cell increase in the placebo group) and a significant (P = .03) difference in weight gain in the ZDV patients aged more than 30 years (8 pounds) as compared with the older placebo patients (aged more than 30 years) (2 pounds) at week 24. The findings in the asymptomatic hemophilic patients aged more than 30 years support a useful effect of ZDV, which is similar to observations in the larger study of its use in asymptomatic, nonhemophilic patients.