Neutropenia occurs often among the newborns of women with hypertension, but its cause, mechanism, and clinical consequences have not been adequately studied. Of 72 infants whose mothers had hypertension during pregnancy, 35 (49 percent) had neutropenia, which persisted from 1 hour to 30 days. The disorder was more prevalent among newborns whose growth had been retarded in utero (P less than 0.01), those who had been delivered prematurely (P less than 0.001), and those whose mothers had had severe hypertension (P less than 0.002) or hypertension and the HELLP syndrome (hemolysis, elevated liver enzymes, and low platelets) (P less than 0.01). Kinetic investigations of circulating, marginated, storage, and proliferative neutrophils and their progenitors suggested that the neutropenia was the result of diminished neutrophil production. Noscomial infections occurred during the first 2 1/2 weeks of life in eight (23 percent) of the newborns with neutropenia, but in only one (3 percent) of those without this disorder (P less than 0.01). We conclude that the neonatal neutropenia associated with maternal hypertension is due to transiently reduced neutrophil production and is associated with an increased risk of noscomial infection. Its basic cause remains unknown.