Amplification of the HER-2/neu oncogene was assessed in 80 cases of epithelial ovarian tumors using differential polymerase chain reaction. HER-2/neu gene was amplified in 22 of 46 invasive cancers (48%) and in 5 of 34 borderline cancers (15%), but none of the 20 specimens of normal ovaries showed amplification. THis difference is statistically significant (p = 0.00004). The incidence of HER-2/neu amplification in late stage (III-IV, 77%) was significantly higher than that in early stage (I-II, 21%) in invasive epithelial carcinoma (p = 0.0004). There was no correlation between HER-2/neu amplification and cell type or grade of tumor. In cases of ovarian tumors of borderline malignant potential, the amplification of HER-2/neu was not correlated with clinicopathologic features. Follow-up with a mean of 22 months (6-50 months) was available for 39 cases of invasive ovarian cancers and all 34 borderline ovarian cancers. The incidence of HER-2/neu amplification in the invasive cancer and borderline cancer patients who were alive with disease was 50 and 50%, and is not statistically different from that in the patients who were alive with no evidence of disease (p = 0.662 and 0.345, respectively). The incidence of amplification in the invasive cancers of patients who died of the disease (86%) was higher than that in the patients who were still alive (44%), but the difference is not statistically significant (p = 0.175). This study supports the association of HER-2/neu amplification with progression of invasive ovarian cancer. It also suggests that HER-2/neu amplification may be an adjunctive prognostic factor of invasive epithelial ovarian cancer, shown to be associated with an unfavorable clinical course. In addition, HER-2/neu amplification occurs relatively infrequently in early invasive and borderline ovarian cancers, making it unlikely that such amplification is a general early event in ovarian carcinogenesis.