AbstractClonal karyotypic abnormalities were detected in 7 of 42 cytogenetically analyzed benign ovarian tumors. An adenofibroma had –X and a mucinous cystadenoma had t(1;11)(q25;q23) as the sole abnormality. Trisomy 12 was found in the remaining five tumors. It was the only change in two fibromas and a serous cystadenoma; the fourth tumor, a mucinous cystadenoma, had one clone with + 12 and one with + 12 and + 10, and the fifth tumor, a fibrothecoma, had +4, +9, +12. The finding of trisomy 12 in five of seven karyotypically aberrant tumors suggests that this aberration characterizes a hitherto unrecognized cytogenetic subgroup of benign ovarian neoplasms.