The clinicopathologic features of 44 serous borderline tumors (SBT) of the ovary were evaluated. Nineteen were Stages II and III, and 9 had invasive peritoneal implants. All 19 patients received chemotherapy and 4, who had invasive implants, died of disease after 3, 4.3, 8, and 9 years. The other 25 patients were free of tumor 1-14 years (mean, 5.3 years) after surgery. Coexpression of low molecular weight keratins (AE1, CAM 5.2) and vimentin was found in all tumors and their implants. No significant differences were found between SBT with different volume-corrected mitotic indices (M/Vi) with respect to gross features, presence or absence of implants, stage, and survival. Cytometric DNA analysis also was performed on the primary ovarian tumors and the implants. Twenty-one primary tumors had diploid or tetraploid histograms, and 23 had aneuploid histograms. DNA ploidy of the primary ovarian tumors did not correlate with gross features, the presence or absence of implants, M/Vi, stage, and survival. The data from this study confirm that most SBT are clinically benign, but SBT with invasive implants may behave aggressively. Expression of intermediate filaments, M/Vi, and DNA ploidy evaluation of the primary ovarian tumors seem to be of no value in predicting prognosis. However, four of seven patients with aneuploid DNA implants died of tumor.