We describe the occurrence of a liver adenoma in a young patient with Klinefelter's syndrome, diagnosed by classic 47,XXY karyotype in all investigated cells and a sex hormone imbalance. To our knowledge, this is the first report of such an association, which might suggest a simple coincidence. However, a pathogenetic link between Klinefelter's syndrome and liver adenoma can be suggested in view of the following facts: Liver adenoma mostly affects women of child-bearing age using oral contraceptives. It is very rare in men, and affected men frequently have been treated with androgens. Both groups are characterized by a sex hormone imbalance. Also, the risk of breast cancer, another hormone-sensitive tumor, is greatly elevated in men with Klinefelter's syndrome, possibly due to sex hormone imbalance.