Fifty-three pairs of twins were obtained during examination of 1,939 spontaneously aborted complete embryos and fetuses. Therefore, the rate of twinning in pregnancies which spontaneously aborted was about 1 in 35. The spontaneous twin abortions consisted of 25 sets of twin embryos, 26 sets of twin fetuses, and two sets in which one was an embryo and one was a fetus. It was possible to determine the zygosity in 37 pairs. The ratio of monozygotic to dizygotic was 17.5:1. About 88% of the twin embryos and 21% of the twin fetuses were abnormal. These abnormality rates were similar to those observed in the overall study of the abortuses. The data on spontaneously aborted twins were compared with data on liveborn twins. The results suggest that twinning, particularly MZ twinning, occurs more frequently than has been thought, based on newborn data, and that embryonic and fetal mortality is much higher in twins than in singletons.