Cytogenetic analysis of short-term cultures from 69 cases of fibrocystic breast changes and 10 samples of normal mammary tissue revealed clonal chromosome aberrations in six fibrocystic lesions. All the histologically normal tissue samples had a normal karyotype. The frequency of cytogenetically abnormal cases seems to correlate with the degree of histopathologic changes of the tissue; nonproliferative lesions may have clonal chromosome alterations, but at a low frequency. Whether women with karyotypically altered fibrocystic "disease" have a higher risk of developing invasive breast cancer, compared with women without microscopically visible genetic anomalies in fibrocystic lesions, remains unknown.