Cachexia is a common problem in persons infected with the human immunodeficiency virus (HIV). Megestrol acetate, an agent used for the treatment of metastatic breast cancer, is associated with appetite stimulation and weight gain. To determine whether this drug might benefit HIV-positive patients, 22 such subjects (14 previously reported) were treated with oral megestrol acetate, beginning at a dose of 80 mg four times daily. All patients had lost at least 10% of their preillness weight prior to treatment; the median loss was 11.4 kg (range, 5.5 to 26.8). Preliminary data from patients observed during therapy from 2 to 72 weeks showed that 21 of the 22 patients gained weight; the average weight gain was 7.3 kg (range, -4.1 to 17.3). Three patients failed to gain weight on 320 mg per day of megestrol acetate; both appetite stimulation and weight gain were achieved with 460 mg per day in one and 640 mg per day in another. One patient continued to lose weight despite 480 mg per day megestrol acetate. The median time to peak weight during megestrol acetate treatment was 14 weeks. Seven patients returned to within 1 kg of their normal body weight. In three of the 22 patients treated, megestrol acetate and zidovudine were started simultaneously. For these three patients, weight gain was potentially due to the recognized weight gain associated with the initiation of zidovudine. For the remaining 18 patients, however, appetite stimulation and weight gain were a result of megestrol acetate. All patients tolerated the drug well. One patient developed a deep vein thrombosis. No patient developed peripheral edema or drug-related impotence. The appetite improvement and weight gain seen in this initial series are encouraging. The true effectiveness of megestrol acetate for HIV-related cachexia and the effects of treatment on quality of life are currently being assessed in a national prospective, randomized, double-blind, placebo-controlled trial.