Placentae from Class C diabetic mothers were compared by histomorphometric analyses with a group of normal placentae. The placentae of the diabetics were divided in two groups based on the growth characteristics and neonatal outcome of the infants at birth. This study has demonstrated that the placentae of both groups were somewhat heavier than the controls due to a parallel increase in parenchymal and non-parenchymal tissues. The placentae were also shown to be characterized by a relative increase in the surface areas of exchange between mother and fetus, in terms of peripheral villous and capillary surface areas and intervillous space volume. Consequently, the results of this study suggest that, in Class C diabetics, placental morphology and placental function are probably not more adversely affected than in other less severe forms of the disease during pregnancy. Furthermore, the findings in this investigation support the hypothesis that the placental changes, and the perinatal morbidity associated with this condition, are probably the results of hormonal and metabolic abnormalities present in the mother and the fetus.