Chromosome studies were successfully carried out on histologically identified tissues from 30 of 67 spontaneously aborted and eight full term products of conception. The anatomical structure of the amnion and chorion was normal in the eight full term cases and in 27 of the aborted specimens, each of which was found to have a normal chromosome complement. In three aborted specimens there were placental abnormalities consisting of hydatidiform degeneration of the chorionic villi, chorionic cysts and arborizing amniotic polyps. Giant cells were scattered through the bases, stems and dilated tips of the amniotic polyps. These abnormal cases had triploid XXY/diploid XY, triploid XXX and triploid XYY karyotypes respectively. In one specimen cultured cells from both the amnion and the embryo yielded triploid XXX karyotypes. The mean nuclear diameter of triploid amnion cells was found to be significantly greater than that of diploid amnion cells (P = 0.001) in the fresh tissue state and in tissue culture.