Placental mesenchymal dysplasia is a rare human placental disorder in which the placenta is enlarged and contains cystic villi and dilated vasculature. It is important to recognize this disorder because it may mimic a partial hydatidiform mole. In contrast to a partial mole, mesenchymal dysplasia may coexist with a normal fetus. This case occurred at the Madigan Army Medical Center in Tacoma, Washington, in June 2002.