This study was performed to evaluate the correlation between HER-2/neu gene expression and DNA ploidy patterns. Forty-five cases of breast-cancer were analyzed. Immunohistochemical staining of HER-2/neu protein on frozen sections was used to detect the HER-2/neu protein, and the Feulgen DNA staining method was used to assess DNA amounts in the same tumor cells. Positive HER-2/neu overexpression was evaluated visually, and quantitation of the HER-2/neu protein was measured by image analysis. Twenty-two of the 45 cases were visually scored to be positive for the overexpression of the HER-2/neu protein, and these cases also contained above 10% HER-2/neu protein compared with a standard control cell line. All 22 of these cases had near-tetraploid DNA content. In contrast, cells, derived from the 23 cases that did not overexpress the HER-2/neu protein, contained DNA amounts that ranged from euploid (diploid) to varying degrees of aneuploid. The results of this study indicated that tumors that overexpress the HER-2/neu protein have tetraploid or near-tetraploid DNA content. This pattern could relate to the biological behavior of these tumors.