We have used the polymerase chain reaction (PCR) technique for the diagnosis of hydatidiform mole, a trophoblastic disease. For this, we targeted the hypervariable 3' flanking region of the APOB gene (APOB/VNTR) because of its high heterozygosity index (0.61) in the Japanese population. We examined seven clinical cases which were tentatively diagnosed as hydatidiform moles. Five of these revealed DNA segments unique to the paternal APOB allele, allowing us to diagnose a complete mole. The PCR technique for targeting the APOB/VNTR appears useful for early diagnosis of hydatidiform mole.