Cytogenetic analysis was performed on 11 benign and borderline ovarian tumors. Trisomy 12, identified as a sole abnormality in 6 tumors, is likely a specific karyotypic change in different benign and borderline tumors and may well be a primary chromosomal lesion in these tumors. The possible association between amplification of the proto-oncogene K-ras-2 which is located on chromosome 12 and trisomy 12 was investigated. DNA blotting analysis of 64 tumors indicates that trisomy 12 does not seem to be related to K-ras-2 amplification in ovarian tumors. K-ras-2 amplification was observed in 3 high-grade tumors from 3 patients with metastases.