In a prospective study sixty-eight patients consecutively diagnosed as having AIDS or advanced ARC who were started on zidovudine therapy were followed up for a median period of 725 days. In the 20 patients who had a baseline p24 antigen level above 20 pg/ml, there was a statistically significant trend towards reduction of the p24 antigen levels after the first month of treatment. The median time of survival of the 68 patients was 702 days and the median symptom-free period was 510 days. Treatment with zidovudine significantly reduced the p24 antigen levels. However, the life expectancy and the symptom-free period were not statistically different in the patients with p24 antigen levels always below or with levels always above two arbitrarily chosen cut-off points of 20 pg/ml and 50 pg/ml, respectively.