Smoldering multiple myeloma (SMM) carries a 50% risk of progression to multiple myeloma (MM) or related malignancy within the first 5 years following diagnosis. The goal of this study was to determine if high levels of circulating plasma cells (PCs) are predictive of SMM transformation within the first 2-3 years from diagnosis. Ninety-one patients diagnosed with SMM at Mayo Clinic from January 1994 through January 2007, who had testing for circulating PCs using an immunofluorescent assay and adequate follow-up to ascertain disease progression, were studied. High level of circulating PCs was defined as absolute peripheral blood PCs >5 × 10(6)/l and/or >5% PCs per 100 cytoplasmic immunoglobulin (Ig)-positive peripheral blood mononuclear cells. Patients with high circulating PCs (14 of 91 patients, 15%) were significantly more likely to progress to active disease within 2 years compared with patients without high circulating PCs, 71% versus 24%, respectively, P=0.001. Corresponding rates for progression within 3 years were 86% versus 34%, respectively, P<0.001. Overall survival (OS) after both SMM diagnosis and MM diagnosis was also significantly different. High levels of circulating PCs identify SMM patients with an elevated risk of progression within the first 2-3 years following diagnosis.