Testicular teratoma in a patient with Klinefelter's syndrome is described. Relationship between chromosomal abnormalities and the risk for neoplasm is discussed. A number of reports on the asoociation between chromosomal abnormalities and neoplasm have been done, and it is well known that Klinefelter's syndrome is prone to combine with breast cancer. The present report describes a patient with Klinefelter's syndrome who had mature teratoma in the testis.