A partial hydatidiform mole differs morphologically and cytologically from the complete mole. Correct preoperative diagnosis of the partial mole by ultrasonography is often difficult. Some partial moles have sonographic appearances indistinguishable from those of the common complete moles or missed abortions. The predominant feature in the remainder is the presence of a well-defined gestational sac, either empty or containing inappropriately small fetal echoes, surrounded by a relatively thick rim of placental echoes with intermingling cystic spaces. An echogenic rim around the sac, as is found in missed abortion or blighted ovum, is notably absent. There appeared to be no correlation between these sonographic patterns and the clinical profiles of patients.