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 cases of missed abortion with placental hydropic degeneration, collected at Taegu Catholic General hospital, from January 1986 to December 1989. The results were as follows: 1. Of 12 cases of complete mole, all demonstrated diffuse intrauterine vesicular pattern of internal echo without a gestational sac. Two cases were recurred after D & E. 2. The partial mole was characterized by focal (70%) or diffuse (20%) distribution of hydatidiform placental change and a gestational sac (100%) with or without a macerated fetus. But the striking hydatidiform placental change was not -present in one case of partial mole. 3. The uterus was larger for dates in 9 cases (90%) of complete mole, but smaller for dates in 7 cases (70%) of partial mole. 4. The missed abortion with plac ntal hydropic degeneration was in distinguished from a partial mole due to their similar sonographic appearance: focal or diffuse cystic change of a placenta, a distorted gestational sac with or without 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: a gestational sac or an area of noncystic placenta was not identified in a complete mole. The partial mole was in distinguished form a missed abortion, but if there is the suspicion of trophoblastic proliferation, such as a convex placental surface or a larger uterus for dates, then the diagnosis is probably a partial mole rather than a missed abortion.