Previous reports have shown that serum beta-2-microglobulin (S beta2M) is a reliable marker of presenting tumor mass, response to chemotherapy, and prognosis of patients with multiple myeloma (MM). In order to more thoroughly evaluate the optimal use of S beta2M in plasma cell dyscrasias (PCD), S beta2M levels were serially measured in 160 patients with MM, in comparison with 37 normal controls (NC) and 28 patients with monoclonal gammopathy of undetermined significance (MGUS). In MGUS, S beta2M did not differ significantly from that of NC, but was significantly lower than that of MM (p less than 0.001), including low cell mass MM (p less than 0.02). In MM, S beta 2M was highly correlated with the total body burden of myeloma cells as derived from the staging of Durie and Salmon, both at diagnosis and in remission (residual tumor mass) (p less than 0.001). During the plateau phase, S beta 2M remained very stable and was always within the normal range for patients with greater than or equal to 75% tumor regression. The most striking finding was that S beta 2M gave an extremely reliable fit for survival prediction at (1) diagnosis, (2) remission, and (3) early relapse, with higher S beta 2M levels in each instance being in favor of poorer prognosis. We conclude that S beta 2M is an extremely useful marker in initial stratification and follow-up of patients with MM.