BACKGROUND Type I collagen is a major constituent of the interstitial connective tissue. Although ovarian carcinoma is known to induce the expression of type I collagen in the peritoneal cavity, the distribution and metabolic activity of this collagen in ovarian tumor tissue are not known. METHODS The distributions and staining intensities of different molecular forms of type I collagen in ovarian neoplasms were studied immunohistochemically with antibodies to the aminoterminal propeptide of type I procollagen (PINP) and the cross-linked carboxyterminal telopeptide of type I collagen (ICTP), reflecting the presence of newly synthesized and old, cross-linked type I collagen, respectively. RESULTS A regular pattern of moderately staining, relatively uniform fibers was observed in the stroma of benign serous and mucinous cystadenomas, indicating limited participation in tumor growth. The staining was accentuated subepithelially in borderline epithelial neoplasms and in well differentiated cystadenocarcinomas, suggesting induction of the stromal collagen synthesis by the tumor cells. Fewer degraded collagen fibers were found in moderately differentiated carcinomas, most likely because of enzymatic degradation of the stroma surrounding the neoplasms during tumor spread. Strongly staining, irregular collagen fibers occurred closely around islets of tumor cells in undifferentiated malignant neoplasms and in metastases of ovarian carcinomas; also, intracellular staining was present in part of the malignant cells. In most cases, the staining reactions obtained with the two different antibodies were similar, probably indicating rapid processing of the newly synthesized type I collagen (indicated by PINP) to a maturely cross-linked form (indicated by ICTP). CONCLUSIONS Synthetic and degradative processes are typical of the collagenous matrix in malignant ovarian tumors. Aberrant expression of type I collagen may occur in anaplastic ovarian carcinomas.