Bilateral breast tumors, a malignant phyllodes tumor in the right breast and an invasive lobular carcinoma in the left breast, occurred in a 47-year-old woman with 46XX/46XY mosaic karyotype in her peripheral blood lymphocytes and intersex external genitalia. Postmortem examination revealed bilateral ovotestis. Three of the patient's sisters also had breast cancer. In situ hybridization with a Y-specific probe revealed Y-chromosome-specific signal in both tumors, suggesting that the clonal origin of tumors in this patient was Y-containing cells. Androgen-receptor polymorphism also revealed a monoallelic X chromosome pattern in the recurrent phyllodes tumor tissue taken at autopsy, in addition to loss of heterozygosity demonstrated at locus TP53. The slippage of the CA repeats in the tumor was also shown at the loci of D5S82 and D11S527. The mechanistic basis for the occurrence of bilateral malignant tumors of the breast, XX/XY mosaicism, and familial clustering of breast cancer is still unknown. The present study, however, suggests that the sex chromosome abnormality may have modified the cancer phenotype in a manner similar to breast cancer in Klinefelter's syndrome (though phenotypically male) and the Y chromosome may have promoted cell growth.