All 190 cases of gestational trophoblastic neoplasia diagnosed in the Baltimore metropolitan area from 1975-1982 were identified. Histologic slides were requested and reviewed independently by two pathologists who agreed upon uniform criteria for the diagnosis of hydatidiform (complete) mole, invasive mole, and choriocarcinoma. A representative sample of the slides was selected and resubmitted to one of the study pathologists for a second review. The inter- and intra-pathologist variability in the diagnosis of gestational trophoblastic neoplasia was calculated using the kappa statistic (K). Our findings indicated that the variability in the diagnosis of gestational trophoblastic neoplasia was low whereas that for the related tumor of incomplete mole was high.