In a 16-mo-old infant born to an HIV-infected mother, repeatedly negative results of a HIV rapid antibody test had been reported during the past 6 mo. The infant presented with several HIV-defining illnesses and HIV RT-PCR testing confirmed the presence of HIV infection. There are at least 2 possible explanations for the child's false-negative rapid HIV test results: First, his primary antibody production may have been suppressed by the presence of maternal IgG antibodies. Second, his mother was highly immunosuppressed, so that the low level of maternally derived IgG was only detected by HIV-EIA and Western blot. Our data suggest that the HIV rapid antibody test may not be sufficiently sensitive to detect HIV antibodies in infants aged <18 mo.