Pediatric acquired immunodeficiency syndrome (PAIDS) results from infection with human immunodeficiency virus (HIV). The majority of infants with AIDS are infected in utero. In most instances, mothers are infected from intravenous drug abuse or bisexual partners. Infected individuals, both mothers and infants, may be asymptomatic for prolonged periods of time. Initially viral replication may be limited and the virus integrated into immune cells. Activation of the virus may result from several factors including infection with other viral agents, such as cytomegalovirus and Epstein-Barr virus. Variable progression of disease may be related to dose of virus, the degree of pre-existing immunodeficiency, and the presence of other infectious agents. Recent evidence suggests that individual HIV isolates vary in their capacity to infect cells. This may explain differences in clinical presentation and progression. A diagnosis of HIV infection may be difficult in infants who have persistence of maternal antibody. Viral isolation is often difficult and unreliable, even in infants who have clinical features of AIDS. Currently only azidothymidine has been shown to reduce viral replication and improve prognosis. However, azidothymidine does not eradicate HIV and patients require repeated or chronic therapy. The drug is not yet approved for use in children, although studies are being performed at several medical centers. Prevention of HIV infection is the single most important approach to controlling its rapid spread in infants. Screening of all blood products by using HIV antibody testing is essential. Increased effort in preventing drug abuse and in reducing pregnancy rates among infected women should be a priority of health care workers.