During 20 months 49 AIDS patients treated with zidovudine were followed prospectively. The 12-month cumulative probability of survival was 73% and the 18-month probability of survival was 51%. The probability of survival was significantly higher when, at the start of therapy, the Karnofsky score was 70 or higher (p less than 0.001) or the CD4 cell count was 0.05 x 10(9)/l or higher (p less than 0.05). The general condition, Karnofsky score, body weight, number of CD4 positive cells and the lymphocyte stimulation in vitro improved during therapy, but the beneficial effects lasted only 6-9 months. Anaemia (Hb less than 6 mmol/l) developed in 21 (43%) of the patients. (Pan)cytopenia prompted dose reduction in 14 patients, in 5 patients with pancytopenia therapy was withdrawn. The length of stay in hospital was 885 days for the whole group of patients, equivalent to 20 days per patient year.