Implicit in the New York State Newborn HIV Seroprevalence Study is the assumption that newborns of all New York State residents are tested for human immunodeficiency virus (HIV) antibodies. We examined this assumption by describing that part of the 1988 New York newborn population not tested in the HIV seroprevalence study and assessing any bias contributed by this group. Of the expected total HIV specimens 1.5 percent were never received by the Newborn Screening Program, 0.5 percent were invalid specimens for which no repeat specimen could be obtained, and 1.7 percent were unsuitable or of insufficient quantity to be tested for HIV antibody. Thus 96.3 percent of all 1988 New York newborns were tested for HIV antibody. Black infants from New York City and low-birthweight infants were represented disproportionately among those not tested. Assignment of all untested newborn to HIV-positive status increased the seroprevalence rate 17 percent (0.64 percent to 0.75 percent).