Partial hydatidiform mole (PHM) is a distinct entity, a triploid, diandric conceptus the placenta of which shows focal trophoblastic hyperplasia (a sine qua non of diagnosis) and focal hydatidiform change; the fetus usually survives until eight to nine weeks' menstrual age. The clinical presentation is varied and preevacuation diagnosis often difficult. No choriocarcinoma has been documented in association with PHM, but residual, nonmetastatic disease can occur. The diagnosis rests ultimately with the pathologist, who has to distinguish PHM from a common abortus with villous edema and from twins in which the partners are a complete mole and a normal conceptus.