It is well recognized that genetic alterations within oncogenes, tumor suppressor genes, DNA mismatch repair and excision repair genes contribute to tumorigenesis within the human ovary. This study was undertaken to screen for the existence of K-ras gene point mutations in paraffin-embedded slides randomly selected from benign and malignant ovarian tumors applying the PCR-RFLP technique. Expression of p21ras was also assessed in 30 primary ovarian adenocarcinomas immunohistochemically. K-ras codon 12 point mutations occurred in nine of 40 (22.5%) cases. They were not identified in two benign mucinous cystadenomas, but in one out of two (50%) mucinous tumors of LMP (low malignant potential), in five out of 30 (17%) ovarian adenocarcinomas, and in one case of adenocarcinoma metastatic to the ovary. K-ras activation was also detected in one out of four (25%) sex cord-stromal cell tumors (folliculoma), and in one dysgerminoma. None of these tumors exhibited K-ras codon 13 point mutations. Gene alterations were more frequently found in mucinous than in non-mucinous (30% vs 10%) tumors, although the difference did not reach significance (p > 0.05). The frequency of K-ras point mutations was correlated neither with clinical nor with pathological variables of cancer. Cytoplasmic p21ras was expressed in all adenocarcinomas negative for K-ras point mutations, whereas one of five (20%) K-ras-positive tumors exhibited lack of immunoreactivity. In conclusion, these findings confirm the role of K-ras activation in mucinous ovarian tumors. p21ras expression is not necessarily associated with K-ras gene alterations in human ovarian adenocarcinomas.