A patient with an intermediate state of human T lymphotropic virus type I (HTLV-I) infection and in whom autopsy showed multiple organ failure (MOF) associated with extensive metastatic calcification in systemic organs is described. A 56-year-old man presented with signs and symptoms of advanced cardiac insufficiency, respiratory disturbance and renal failure. Serologically, the anti-human T lymphotropic virus type I (HTLV-I) antibody titer and the levels of both calcium and parathyroid hormone-related peptide (PTHrP) were distinctly elevated. These data suggested a diagnosis of adult T cell lymphoma/leukemia (ATLL). However, examination of a peripheral blood sample revealed only a few atypical lymphoid cells (3%) associated with mild leukocytosis (white blood cell count, 13.7 × 103/mm3). Lymph node swelling was systemic but mild, with some nodes up to 10 mm in diameter. The patient died of MOF. Adult T cell leukemia/lymphoma was unable to be diagnosed definitively because of the short duration of laboratory abnormalities and because of the discrepancy between the laboratory data and the magnitude of lymphoproliferation in both the lymph nodes and peripheral blood. At autopsy, the most conspicuous finding was extensive metastatic calcification in the multiple organs, including the heart, lungs, kidneys, tongue, liver, pancreas, spleen and systemic arterial walls. Very small numbers of medium-sized atypical lymphoid cells admixed with small reactive lymphocytes were identified in multiple organs, with no evidence of massive infiltration. Molecular analyses could not detect monoclonal integration of HTLV-I provirus DNA or monoclonality of T cell lineage cells. Parathyroid hormone-related peptide was demonstrated in the cytoplasm of the atypical lymphoid cells on immunohistochemical examination. The bone trabeculae generally showed distinct evidence of resorption associated with marked proliferation of osteoclasts. These findings suggested that the hypercalcemia in the present case was categorized as humoral hypercalcemia of malignancy rather than local osteolytic hypercalcemia.