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.