Malignant ovarian tumors induce a strong fibro-proliferative reaction characterized by the active production of type I and type III procollagen both locally in the ovary as well as more remotely in the peritoneal cavity. Our purpose was to determine the origin of the increased collagen production observed in serous ovarian tumors with different histological grades of malignancy, ie, whether the malignant cells or the stromal fibroblasts are responsible for the synthesis of collagen fibers. We visualized the mRNAs corresponding to the pro alpha 1(I) and pro alpha 2(I) chains of type I procollagen and the pro alpha 1(III) chain of type III procollagen by in situ hybridization. Strong signals for both chains of type I procollagen were seen in stromal fibroblasts next to tumor cell islets, whereas the reaction was weak or absent near benign ovarian cysts. In poorly differentiated tumors, the signals were particularly abundant and occasionally also seen in the neoplastic cells themselves. Type III procollagen mRNA expression was similar, although somewhat less distinct. These findings indicate that the production of interstitial procollagens is related to the degree of malignancy and neoplastic activity of tumors. The formation of collagen in well differentiated ovarian tumors is a function of stromal fibroblasts, whereas in poorly differentiated tumors, aberrant expression of one or several chains of type I and type III procollagens in the neoplastic cells is also likely to take place.