Zidovudine (AZT) is of some benefit for selected patients with AIDS-related complex (ARC) or AIDS treated for up to 24 weeks. The activity and toxicity of oral AZT, 200 mg 4-hourly when possible, was evaluated in 365 consecutive patients with ARC (80) or AIDS (285) followed up for a mean of 31 weeks (range 2-52). A transient increase in body weight, Karnofsky index, and CD4 cell count was observed during the first months of therapy. However, by 6 months, these values had returned to their pretreatment levels and several opportunistic infections, malignancies, and deaths occurred. These disappointing results were partly related to the haematological toxicity of the drug, which led to interruption of treatment in many patients. Thus the benefits of AZT are limited to a few months for ARC and AIDS patients. At least for the most severely affected patients, reduced dosage of AZT may increase the therapeutic index.