The effects on the neonate of severe maternal hypertension originating before the thirty-sixth week of gestation were determined by comparing data obtained on 28 preterm infants born of hypertensive mothers with data from 28 gestational age-matched controls. All hypertensive mothers had diastolic blood pressure greater than or equal to 110 mm Hg, proteinuria, and systemic symptoms of their disease; over half had thrombocytopenia and significant elevations of LDH and SGOT. All hypertensive mothers had been treated intravenously with magnesium sulfate, and 79% received other antihypertensive agents. When compared to control infants, the infants of hypertensive mothers had a significantly higher incidence of somatic growth retardation, microcephaly, thrombocytopenia, leukopenia, neutropenia, low Apgar scores, delayed adaptation, patent ductus arteriosus, hypotonia, and gastrointestinal hypomotility. Apgar scores, platelet count, WBC count, neutrophil count, and weight percentile correlated with the severity of maternal platelet and enzyme abnormalities. The occurrence of gastrointestinal hypomotility, hypotonia, and patent ductus arteriosus may be related to transplacental passage of maternally administered drugs.