Infants born to women with hematocrit values below 31% showed an increased frequency of low birth weight. However, when the women were stratified by smoking history, differences in birth weight became apparent at the higher hematocrit levels. The data suggested an increased risk of low birth weight among women who smoke and have hematocrit values in the 41--47 range. Since smoking alters plasma volume, care should be taken when evaluating hematocrit levels measured late in pregnancy.