The subject of this retrospective study was to evaluate the potential benefit of the c-erbB-2 oncogene amplification and expression in 27 complete hydatidiform moles as well as in 9 cases of persistent gestational trophoblastic disease defined by elevated serum beta-human choriongonadotropin. The persistent cases were histopathologically classified as 5 complete hydatidiform moles, 3 invasive moles and 1 choriocarcinoma. In addition, we determined the DNA content and the karyotype of the sex chromosomes. The data were correlated with the histopathologic characteristics of gestational trophoblastic diseases. Cases with c-erbB-2 amplification and expression in combination with DNA hyperploidy showed higher proliferation and a more aggressive behavior (2 complete hydatidiform moles with lung and liver metastases, 2 invasive moles and 1 choriocarcinoma). XY karyotype was evident in the choriocarcinoma and in two complete hydatidiform moles with advanced stage and DNA hyperploidy.