Acquired immune deficiency syndrome (AIDS) has afflicted persons of all ages, yet only recently has attention been devoted to AIDS in older persons. To examine the epidemiology of AIDS in persons greater than or equal to 50 years old in the United States, we analyzed cases reported to the Centers for Disease Control. The number reported annually in persons greater than or equal to 50 years old increased from 13 in 1981 to 3,562 in 1989. Through December 1989, 11,984 had been reported, representing 10% of all cases. Although male homosexual contact accounted for most cases in persons aged 50-69, blood transfusion became a more common means of exposure with increasing age, accounting for 28% of cases in persons aged 60-69 and 64% of cases in individuals aged greater than or equal to 70. The proportion of women increased from 6.1% in persons with AIDS aged 50-59 to 28.7% of those aged greater than or equal to 70. The proportion of AIDS diagnoses made in the same month as death increased from 16% in persons aged 50-59 to 37% in those aged greater than or equal to 80, suggesting either more rapid progression of disease or increasing delay in diagnosis. As the incidence in older persons continues to increase, clinicians caring for older patients must become more familiar with AIDS.