The mutation of K-ras protooncogene was examined in 44 cases of borderline ovarian epithelial tumors and 18 cases of invasive ovarian carcinomas. In borderline tumors, K-ras mutations are a common feature, having been found in 21 of 44 cases (48%). Twenty of the 21 mutations were identified at codon 12, and one was identified at codon 13. A detailed analysis of the mutation pattern of K-ras revealed a close association with the histological cell types of the tumor. Mutation of K-ras was detected at a higher frequency in mucinous borderline tumor (identified in 12 of 19 cases) compared to serous borderline tumor (identified in 9 of 25 cases). K-ras mutation was also detected in invasive mucinous and serous ovarian carcinomas, hence supporting the notion that borderline ovarian tumors may represent a pathological continuum between benign and frankly invasive diseases.