A quantitative morphologic study has been made of body size, organ size, and cellular structure in 23 newborn infants with erythroblastosis fetalis. Spleen, liver, heart, and adrenals were enlarged for gestational age while thymus, brain, and lymph nodes were subnormal in size. The quantitative analysis revealed a hyperplasia of myocardial fibers, of pancreatic islet cells, and of fetal zone adrenal cortical cells. Islet cells and adrenal cortical cells were hypertrophic while cells in other organs had normal dimensions. Individual aspects of these morphologic abnormalities resemble features of other disorders and in some instances a similar pathogenesis may exist.