Free light chain (FLC) assays are important in the diagnosis and monitoring of patients with multiple myeloma (MM). Serum FLC concentrations also correlate with disease course in the majority of MM patients and have been incorporated into the new response criteria. Although baseline values of FLC are prognostic in newly diagnosed MM, serial measurement of serum FLC does not appear to be of greater value in patients who have measurable M-proteins by electrophoresis. We examined the kinetics of serum FLC in six patients with newly diagnosed MM during treatment with high-dose dexamethasone (HD-DEX) and bortezomib and dexamethasone. In some cases, the involved serum FLC increased in the latter part of each chemotherapy cycle before the start of the next cycle, especially in HD-DEX, suggesting that the response to these agents may be insufficient for induction therapy for MM. Earlier disease assessment by serum FLC assays may be of value in detecting poorly responding patients who require alternative forms of therapy.