The chromosomal composition of cancers arising in ovarian mature cystic teratoma (MCT) was analyzed using chromosome in situ hybridization (CISH) performed on paraffin-embedded material from 10 squamous cell carcinomas, 1 adenocarcinoma, 1 thyroid papillary carcinoma, and 1 strumal carcinoid. Cervical tissue from seven of these patients was used as a control. In the cancers, a relative gain of chromosome 16 (in 7 cases), chromosome 12 (in 9), chromosome 11 (in 10), and chromosome X (in 8) was detected. Trisomy 12 was diagnosed in eight cases. No chromosomal aberrations were detected in the strumal carcinoid. In the benign areas of the MCTs, only one case showed a gain of chromosome 11, although, in another case, there was a gain of chromosome X. Malignant transformation of MCTs often occurs in association with complex chromosome aberrations that may play a role in the development of such carcinomas.