Vertically transmitted human immunodeficiency virus is rapidly becoming a major public health problem. Recent investigations have helped clarify the rate of transmission from mother to child and the factors that affect transmission. However, there is still no clear consensus as to one absolute rate, and perhaps never will be. In addition, improved methods of diagnosis will soon enable a more rapid recognition of those human immunodeficiency virus-seropositive infants who are actually infected with the virus, and infants who are antibody positive only because of transferred maternal antibody.