In 33 patients with primary ovarian cancer who had not received any chemotherapy before surgery, expression of glutathione S-transferase pi (GST-pi) was studied immunohistochemically in relation to the response to chemotherapy with CDDP, and furthermore it was examined whether numerical aberration of chromosome 11 was available as a prognostic indicator. The obtained results were as follows: 1. Of 33 ovarian cancer tissue samples, 19 (57.6%) showed positive staining and 14 (42.4%) negative staining for GST-pi. 2. The 5 year survival rate of the patients with GST-pi positive tumors was significantly lower than that of those with GST-pi negative tumors (p < 0.01). 3. The frequency of detection of cells showing numerical aberration of chromosome 11 in tissues was significantly higher in ovarian cancers than in benign ovarian tumors and normal ovaries. 4. The 5 year survival rate of the patients with ovarian cancer in which the cells showing numerical aberration of chromosome 11 at 20% or more was significantly lower than that of those in which such cells were under 20% (p < 0.02). These results show that expression of GST-pi and numerical aberration of chromosome 11 in the tissues of human ovarian cancers are useful as prognostic indicators.