The levels of adenosine deaminase (ADA) were determined in the erythrocytes of 10 patients with sexually transmitted HIV-1 infection [five cases with AIDS-related complex (ARC) and five with AIDS] before and after therapy with zidovudine (azidothymidine; AZT). A linear increase in ADA activity was observed during the second and third months of zidovudine treatment, with a final increase of about threefold after 3 months of drug administration. The concentration of adenosine triphosphate (ATP) was significantly lower in the erythrocytes of the same group of patients with respect to healthy controls, and a further decrease was noted after 3 months of zidovudine treatment. The results obtained indicate that treatment of ARC/AIDS subjects with zidovudine induces metabolic changes which could be responsible for the development of anaemia, an adverse effect frequently associated with zidovudine therapy.