Eighteen placentas from a middleclass North American urban population (Boston) and 20 placentas from a Central American urban population of low socioeconomic status (Guatemala) were examined by morphometric techniques including an image quantizer. Placental mass was 14% less in the Cuatemalan group. Total mass of peripheral villi was decreased 24% with a proportional reduction in trophoblast mass; on the other hand, the mass of the stem villi was only 5% less than in Boston. Villous and capillary surfaces of the peripheral villi were reduced 22% and 38% respectively in the Guatemalan group, while stem villous surfaces and capillary surfaces were actually increased 20% and 40% respectively. No significant differences were found in the cross-sectional areas of villi in the two populations. Because of this similarity of dimensions of average villi in the two groups, the reduction in villous mass and surface areas could only be attributed to a decreased number of villi in the Guatemalan group. Perivillous fibrin was unexpectedly prominent in the Guatemalan placentas and was often associated with mononuclear infiltration, suggesting parenchymal placental disease.