To establish the sonographic characteristics of the hydatidiform mole and the missed abortion with placental degeneration, we have retrospectively analyzed 12 cases of complete mole, 10 cases of partial mole, and 10 casesof missed abortion with placental hydropic degeneration, collected at Taegu Catholic General Hospital, fromJanuary 1986 to Dec. 1989. The results were as follows: 1. Of 12 cases of complete mole, all demonstrated diffuse intrauterine vesicular pataern of internal echo without a gestational sac. Two cases were recurred after D & E. 2. The patial mole was characterized by focal (70%) or diffuse(20%) distribution of hydatidiform placental chante anda gestational sac (100%) with or without a macerated fetus. But the striking hydatidiform placental change was notpresent in one case of partial mole. 4. The missed abortion with placental hydropic degeneration was indistinguished from a partial mole due to their similar sonogaphic appearance: focal or diffuse cystic change ofa placenta, a distorted gestational sac with or wtihout a fetus, and a smaller uterus for dates. On conclusion, the complete mole could be easily distinguished from a partial mole or a missed abortion by sonography : agestational sac or an area of noncystic placenta was abortion, but if there is the suspicion of trophoblastic proliferation, such as a convex placental surface of a larger uterus for dates, then the diagnosis is probably apartial mole rather than a missed abortion.