Solitary plasmacytoma represents a heterogeneous group of patients; approximately half develop multiple myeloma (MM) in 2-3 years, while others remain disease-free at 10-years. By definition, these patients do not have morphological bone marrow (BM) plasma cell (PC) infiltration. Here, we investigated if sensitive BM evaluation of patients with solitary bone plasmacytoma (SBP;n=35) and extramedullary plasmacytoma (EMP;N=29) through multiparameter-flow-cytometry (MFC) would unravel the presence of clonal PCs in otherwise disease-free BM, and if BM clonality predicted higher risk of progression. BM clonal PCs were detected in 17/35 (49%) SBP and 11/29 (38%) EMP patients. 71% of Flow-positive vs. only 8% of Flow-negative SBP patients evolved to MM (median time-to-progression of 26 months vs. not reached; HR:17.4; P<.001). No significant differences were observed among EMP cases. Our results highlight the importance of MFC for sensitive BM evaluation of SBP patients, to predict risk of developing treatment-requiring MM and to plan disease monitoring.