Comparative cytogenetic and histologic studies on 18 mesothelial ovarian tumors revealed a normal chromosome complement in benign lesions, and the well-known cytogenetic pattern in cystadenocarcinomas. But all borderline tumors of the series evidenced an abnormal stem line and a more or less marked tendency to polyploidization. Serous papillary cystadenomas of this group showed in five out of six cases a stem line with the karyotype 47,XX+C10 (identified by Q-banding), present in both sides of bilateral lesions. It is evidenced that malignant change on the chromosomal level precedes histologically detectable features of malignancy. Histologic equivalents appeared, when the abnormal cell line was established. The initiation of malignant transformation therefore may be signalized by karyotype abnormalities before structural changes can be detected in the corresponding histologic specimens. The results discussed include the concepts of multicentric origin and clonal evolution of malignancy.