The gene encoding the alpha subunit of human chorionic gonadotropin contains at least two polymorphic sites in its 3' flanking region detected by restriction enzymes HindIII and EcoRI. We used these polymorphic sites as markers of tissue genotype in normal placenta, hydatidiform mole, choriocarcinoma, and peripheral leukocytes. As expected, inheritance patterns of most hydatidiform moles showed only a paternal genetic contribution. However, one uncommon DNA polymorphism pattern, homozygosity for the absence of the EcoRI site and the presence of the HindIII site, predominated in choriocarcinoma. Thus, our results suggest that moles which have this uncommon polymorphism pattern appear particularly likely to develop into choriocarcinoma.