Medical and technological advances in the care of infants with low birth weight (less than 2,500 grams, or 5 pounds, 8 ounces) and very low birth weight (less than 1,500 grams, or 3 pounds, 5 ounces) have substantially increased the survival rate for these infants and have led to concerns about the demands their care places upon their families and society. The dollar cost of the resources used disproportionately to care for low birth weight children is one measure of the burden of low birth weight. Using analyses of national survey data for 1988 for children ages 0 to 15, this article presents estimates of the direct incremental costs of low birth weight--costs of the resources used to care for low birth weight infants above and beyond those used for infants of normal birth weight. In 1988, health care, education, and child care for the 3.5 to 4 million children ages 0 to 15 born low birth weight between $5.5 and $6 billion more than they would have if those children had been born normal birth weight. Low birth weight accounts for 10% of all health care costs for children, and the incremental direct costs of low birth cost weight are of similar magnitude to those of unintentional injuries among children and in 1988 were substantially greater than the direct costs of AIDS among Americans of all ages in that year.