A total of 34 surgical specimens, obtained from 13 patients with ovarian tumours of borderline malignancy (OTBM), were investigated by conventional histology, immunocytochemistry and DNA cytophotometry. The lesions were obtained by primary ovarian surgery or second-look procedures and altogether comprised 19 (single and bilateral) OTBM, 8 cases of endosalpingiosis, 4 in situ and 2 invasive peritoneal implants and 1 overt adenocarcinoma. The morphological findings were related to follow-up data, which showed neoplasms with clinically malignant behaviour in 2 patients. The histology of the extra-ovarian manifestations was not associated with their immunocytochemical properties or with their DNA content. There were no correlations between the evolution of disease and microscopical features but the clinical course appeared to be linked to the DNA content of the extra-ovarian lesions, which was of greater prognostic importance than DNA ploidy of the ovarian tumours. Recurrence-free survival was noted in all 5 patients with diploid or euploid extra-ovarian proliferations, while the 2 clinically malignant cases fell into the group of 3 patients with noneuploid or aneuploid specimens. DNA estimations may be a methodology which increases the prognostic value of second-look procedures in OTBM patients.