Patients with a 47, XXY karyotype (Klinefelter syndrome) appear to have an increased risk of developing a malignancy in adulthood, usually cancer of the breast, extragonadal germ cell tumor, and acute nonlymphoblastic leukemia. There is growing evidence to show that these patients also have an increased risk of developing a malignancy in childhood. There are reports describing the development of acute lymphoblastic leukemia, retinoblastoma, and rhabdomyosarcoma in children with a 47, XXY or mosaic 47, XXY/46, XY karyotype. We report a child with a bone metastasizing, B-cell lineage, non-Hodgkin's lymphoma (NHL) who was found to have a 47, XXY karyotype in both the tumor and constitutional cells.