Thirty-five patients with ovarian tumors operated on between December, 1989 and June, 1991 were studied to detect K-ras codon 12 point mutation (PM). (1) Five of 35 ovarian tumors (14.3%) disclosed K-ras PM at codon 12 and all the PM cases were in transition from GGT to GAT. On the other hand only one case (5.3%) with K-ras oncogene amplification was found and no C-myc or erbB-2 amplification was detected. (2) The incidence of PM according to clinical stages was seen in 3 of 11 stage I cases (27.3%), in 1 of 3 stage II cases (33.3%), in 1 of 14 stage III cases (7.1%) and in neither of 2 stage IV cases. PM was therefore seen in relatively early stages. (3) The occurrence of PM according to the histologic type was found in 3 of 16 serous tumors (18.8%), in 2 of 5 mucinous tumors (40.0%) and in none of 7 clear cell carcinomas or 2 endometrioid carcinomas. (4) Concerning the relation of PM to the involvement of serosal surface of ovarian tumors and to the ascitic cytology, no particular correlation was observed in our study. (5) Regarding the cytologic findings in imprint smears of the tumors in reference to PM, such as nuclear size, shape, N/C ratio, chromatin pattern, nucleolar size and number, the cases with PM tended to have more multiple nucleoli than PM negative cases. No other findings seemed to indicate the clinical progress of cancer. In conclusion, our study indicated that PM in ovarian cancers was a relatively early event in carcinogenesis.