Complete or 'true' hydatidiform mole, an abnormality of human gestation, is characterized by hydropic degeneraton of all placental villi, marked hypertrophy of the trophoblast, absence of a fetus and a propensity to become malignant. The chromosome constitution of complete moles is usually 46,XX, and Kajii et al. reported that the entire genome in seven out of seven cases was paternal in origin, with all centromere markers homozygous for paternal heteromorphisms. These observations, since confirmed, can be explained by the fertilization of an 'empty' egg--no effective genome--by either a haploid sperm that then duplicates without cytokinesis, to restore the diploid number, or by a diploid sperm resulting from failure of the second meiotic division. We report here a study of a series of complete moles that shows the first alternative to be correct in the majority of cases.