To investigate whether monoclonal plasma cells in the bone marrow are useful as a therapeutic marker in AL amyloidosis, serial flow cytometry was performed in five patients with this disorder before and after chemotherapy. Four patients were treated with 2 or 3 courses of VAD (vincristine, doxorubicin and dexamethazone) and subsequently with high-dose melphalan followed by auto-PBSCT. The remaining one patient was treated with two courses of VAD alone. Before treatment all patients exhibited a CD19- CD56+ subpopulation, which indicated monoclonal plasma cells, in varying degrees. After treatment all patients showed a decrease in monoclonal plasma cells in accordance with the disappearance of M-protein in serum and/or urine. In two patients treated with VAD followed by auto-PBSCT, polyclonal (CD19+ CD56-) and total plasma cells gradually increased in the follow-up study, while monoclonal plasma cells stayed at less than 0.3% nine months after treatment. No apparent correlation was found between altered maturation of plasma cells and disappearance of M-protein. With respect to easy detection of monoclonal plasma cells producing amyloidogenic M-protein, flow cytometry of bone marrow aspirates is useful and reliable in the follow-up of patients with AL amyloidosis and in the evaluation of the effects of chemotherapy.