I analyzed trends in the survival of 36 847 adults who were diagnosed with acquired immunodeficiency syndrome between January 1984 and September 1987 under the pre-1987 surveillance definition of acquired immunodeficiency syndrome. For patients in whom Pneumocystis carinii pneumonia was among the first manifestations of acquired immunodeficiency syndrome, the estimated 1-year survival increased from 42.7% for those diagnosed in 1984 and 1985 (95% confidence interval, 41.5% to 44.3%) to 54.5% for those diagnosed in 1986 and 1987 (95% confidence interval, 53.7% to 55.7%). The gain in survival was observed in homosexual men and intravenous drug users of both sexes, in all age and racial groups, in all geographic regions, and in patients with and without coexisting initial diagnoses. Reduced mortality in the 3-month period immediately following the initial diagnosis of acquired immunodeficiency syndrome contributed little to the overall gain in survival. No gain in survival was seen for patients in whom P carinii pneumonia was not an initial manifestation of acquired immunodeficiency syndrome. It is unlikely that the observed improvements in survival resulted solely from errors in death reporting. Better diagnosis and treatment, particularly the introduction of zidovudine in 1986, may have contributed to the decline in mortality.