We undertook a study to determine whether partial hydatidiform mole could be distinguished from other cases of first-trimester missed abortion using ultrasound. Scans from 22 cases of pathologically proved partial hydatidiform mole and 33 cases of first-trimester missed abortion were independently reviewed by three radiologists, each unaware of the final pathologic diagnosis. Using a standard data form, each radiologist recorded the dimensions, shape, and contents of the gestational sac, the sonographic appearance of the decidual reaction/placenta and myometrium, and the presence or absence of adnexal cysts. The following two criteria were found to be significantly associated (P less than .05) with the diagnosis of partial mole: 1) ratio of transverse to anteroposterior dimension of the gestational sac greater than 1.5, and 2) cystic changes, irregularity, or increased echogenicity in the decidual reaction/placenta or myometrium. There was high interobserver correlation for both criteria, as measured by the kappa statistic. In 50% of the cases, either both or neither of these criteria were met. When both criteria were met, the frequency of partial mole was 87%; when neither criterion was met, the frequency of missed abortion was 90%. These results indicate that ultrasound can be of value in predicting a high likelihood of partial mole prior to curettage.