Humoral hypercalcemia due to parathyroid hormone-related peptide (PTHrP) elevation is a well-known complication of various malignancies, but the situation is rare concerning hematological malignancies except for adult T-cell leukemia/lymphoma. We report a case of Richter's syndrome with humoral hypercalcemia, and demonstrate by reverse transcription polymerase chain reaction (RT-PCR) that peripheral blood PTHrP levels were 2,500-fold higher compared to healthy controls. PTHrP production by tumor cells in chronic lymphocytic leukemia (CLL) and Richter's syndrome has been previously demonstrated by nonquantitative methods such as immunohistochemistry and northern blot analysis, but this is the first report using the RT-PCR method. The presented case did not have hypercalcemia when initially diagnosed as small lymphocytic lymphoma (SLL), and as reported earlier, the development of hypercalcemia may be an indication of the transformation to Richter's syndrome in patients with CLL/SLL.