From 89 cases of invasive mole, collected by our department since 1948, 12 (13.5%) cases without preceding hydatidiform mole (group A) were taken out, and their clinical data and histological observations were studied in comparison with those preceded by hydatidiform mole (group B) in the remaining 77 (86.5%), with the following results: 1. Invasive mole in group A was preceded by artificial or spontaneous abortion, but not by premature or normal delivery. 2. All of the 12 cases developed the invasive mole within 6 months. 3. Relation between the size of the focus and the latency period was not observed in either of the two groups. 4. The incidence of preoperative metastasis was somewhat lower in group A, but no difference was visible between the two groups in prognostic result. 5. As for histological findings, edematization and vesiculation of stroma of villi were observed in all the cases of group A, though with difference in grade from case to case. These findings as well as proliferation and intervascular penetration of villous cells were similar to those of group B.