To estimate the number of youth in the United States who have been or will be left motherless by the human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS) epidemic, in order to project the need for family supports, age-appropriate foster and congregate care, and mental health and social services.Orphans are defined as youth whose mothers (the usual caregiving parent) die of HIV/AIDS-related causes. A mathematical model was constructed to estimate the number of such motherless youth. Cumulative fertility rates were applied to the number of reported AIDS deaths (1981 through 1990) and projected deaths (1991 through 1995) of adult women less than 50 years old. The results were adjusted for underreporting of HIV/AIDS-related mortality, pediatric AIDS deaths, infant mortality, ethnic and racial variation in fertility, and decreased fertility associated with late-stage HIV disease. Estimates were made for the number who were children (less than 13 years of age), adolescents (13 to 17 years of age), or young adults (18 years of age or older) at the time of their mothers' death.By the end of 1995, maternal deaths caused by the HIV/AIDS epidemic will have orphaned an estimated 24,600 children and 21,000 adolescents in the United States; unless the course of the epidemic changes dramatically, by the year 2000, the overall number of motherless children and adolescents will exceed 80,000. In 1991, an estimated 13% of US children and 9% of adolescents whose mothers died of all causes were children of women who died of HIV/AIDS-related diseases. These proportions will surpass 17% and 12%, respectively, by 1995. The vast majority of these motherless youth will come from poor communities of color.A large and rapidly growing number of American youth are being orphaned by the HIV/AIDS epidemic. Unless increased attention and resources are devoted to this vulnerable population, a social catastrophe is unavoidable.