The nuclear DNA content of 20 anembryonic pregnancies was studied by flow cytometry from paraffin embedded tissue blocks. An abnormal amount of DNA content was found in 8 of the cases. This was a significantly higher percentage than encountered in spontaneous abortions studied by the same population (40 and 9%, respectively, p < 0.05). The S-phase fraction in anembryonic pregnancies was lower than in spontaneous abortions (22.4 +/- 12.7 and 35.4 +/- 6.8, respectively, p < 0.01). The results indicated that abnormal embryogenesis with grave chromosomal aberrations may play a major role in the etiology of anembryonic pregnancy.