Human parathyroid hormone-related protein (PTHrP) mRNA was detected in peripheral leukemic cells obtained from adult T-cell leukemia (ATL) patients as well as in cultured human T-cell leukemia virus type I (HTLV-I)-infected T-cell lines. In contrast, PTHrP mRNA was not detected in other types of leukemic cells. Using radioimmunoassay, immunoreactive PTHrP was also detected in the spent media of HTLV-I-infected T-cell lines. These results suggest that PTHrP plays an important role in developing the hypercalcemia frequently observed in ATL patients.