The immunohistochemical distribution of inhibin-alpha in formalin-fixed, paraffin-embedded tissues was evaluated in placentas (2 to 40 weeks of gestation), implantation sites, and a variety of trophoblastic lesions. In the first trimester placenta, inhibin-alpha was strongly and diffusely expressed in syncytiotrophoblast. Implantation site intermediate trophoblast in normal and exaggerated placental sites was either negative or only weakly and focally positive for inhibin-alpha. With increasing gestational age, the staining intensity and distribution of inhibin-alpha decreased in syncytiotrophoblast but increased in the implantation site intermediate trophoblast. Chorionic-type intermediate trophoblast, present in the chorion laeve of the term placenta, was weakly but diffusely positive for inhibin-alpha. Cytotrophoblast remained negative for inhibin-alpha throughout gestation. In trophoblastic lesions, inhibin-alpha immunoreactivity was detected in all 17 hydatidiform moles (7 complete and 10 partial), 32 placental site nodules, 23 placental site trophoblastic tumors, 15 epithelioid trophoblastic tumors, and 16 choriocarcinomas. Inhibin-alpha immunoreactivity was confined to the syncytiotrophoblast in hydatidiform moles and choriocarcinoma. As with the normal placenta, inhibin-alpha was not detected in cytotrophoblast. To evaluate the utility of inhibin-alpha in the differential diagnosis of gestational trophoblastic lesions, we tested 32 squamous cell carcinoma of the cervix, 11 low-grade endometrial stromal sarcomas, 12 endometrial (7 well differentiated and 5 moderately differentiated) carcinomas, 7 epithelioid leiomyomas, and 10 leiomyosarcomas for the expression of inhibin-alpha. None of these lesions was positive. These data indicate that inhibin-alpha is expressed by all populations of trophoblast except cytotrophoblast and in all gestational trophoblastic lesions. Accordingly, immunohistochemical detection of inhibin-alpha is useful in the differential diagnosis of gestational trophoblastic lesions.