Serum B-cell maturation antigen levels correlated with the proportion of plasma cells in bone marrow biopsies (Spearman's rho = 0.710; <i>P</i><0.001), clinical status (complete response <i>vs</i> partial response, <i>P</i>=0.0374; complete response <i>vs</i> progressive disease, <i>P</i><0.0001), and tracked with changes in M-protein levels.