Breast cancer in a patient with Klinefelter's syndrome is reported. Possible correlation between testosterone and estradiol serum levels after testosterone-enanthate substitution, estrogen receptors in tumor tissue and clinical symptomatology are discussed. The various theories of etiology concerning breast cancer in this syndrome are reviewed. The increased conversion of testosterone to estradiol at the therapy with androgens might be responsible for the development of breast cancer in Klinefelter's syndrome. The current way of treatment is described.