Infection with human T-cell leukemia virus type 1 (HTLV-1) leads sometimes to the development of adult T-cell lymphoma/leukemia (ATL), which is invariably fatal and often associated with humoral hypercalcemia of malignancy. The transformation of infected CD4 T cells and the pathogenesis of leukemia have been studied with great limitation in tissue culture and patients. To better understand the pathogenesis and perform preclinical drug studies, animal models of ATL are urgently needed. In mice, inoculation of HTLV-1 cell lines mostly leads to development of localized lymphomas. To develop an ATL animal model with leukemic spread of ATL cells, mouse strains with different well-defined immune deficiencies were inoculated intraperitoneally with different HTLV-1-infected cell lines (ACH.2, C8166, MT-2, MET-1). Inoculation of MET-1 cells into NOD/SCID mice provided the best model system for slowly developing T-cell leukemia with multiple organ involvement. In leukemic mice, an increase in serum calcium levels correlated with expression of receptor activator of nuclear factor kappa-light-chain-enhancer of activated B cells ligand on leukemic cells and secretion of parathyroid hormone-related protein and interleukin-6. In contrast to the other cell lines that did not spread systemically, MET-1 expressed both the adhesion molecules CD11a (LFA-1alpha) and CD49d (VLA-4alpha) and produced or induced expression of matrix metalloproteinases 1, 2, 3, and 9, thus underlining the importance of these molecules in the spread of adult T-cell leukemia cells. The MET-1/NOD/SCID model will be useful for developing interventions against invasion and spread of leukemic cells and subsequent humoral hypercalcemia of malignancy.