We observed a unique case of multiple myeloma that was transformed into plasma cell leukemia presenting with subcutaneous and soft tissue infiltrates. Subcutaneous and minor pelvic soft tissue plasmacytomas and the leukemic transformation were diagnosed in a 72-year-old woman after she had completed 9 months of chemotherapy for IgG kappa multiple myeloma. Immunophenotypic study revealed that leukemic cells in her peripheral blood were positive for ICAM-1 (CD54) but negative for LFA-1 alpha (CD11a) and LFA-1 beta (CD18), whereas infiltrating leukemic cells in the subcutaneous plasmacytoma of the left thigh were positive for LFA-1 alpha and LFA-1 beta but negative for ICAM-1. In addition, intermingling capillary endothelial cells were positive for ICAM-1. Extramedullary soft tissue plasmacytoma is uncommon in association with plasma cell tumors, and the exact mechanism of the development of plasmacytoma is not known. In the present case, however, the discordant expression of LFA-1/ICAM-1 adhesion molecules may have accounted for the distinct patterns of growth and the spread of the subcutaneous plasmacytoma through homing of the LFA-1 alpha+, LFA-1 beta+ leukemic cells to ICAM-1+ endothelial cells.