Fourteen placentas from pregnancies complicated by insulin-dependent diabetes mellitus have been examined by quantitative morphometry. The results have been compared with those from 22 placentas of comparable gestation from uncomplicated pregnancies. The volume of parenchymatous tissue in the placentas from diabetic mothers was significantly increased while the volume of nonparenchyma was decreased. The villous surface area was increased in placentas from the diabetic group, the mean value being 17.3 m2 compared with the 11.4 m2 of the normal group. This increase was larger than would be expected when the increase in fetal weight of some babies born to diabetic mothers is taken into account.