Abstract Purpose: Genetic aberrations detected in multiple myeloma (MM) have also been reported in the premalignant conditions monoclonal gammopathy of undetermined significance (MGUS) and smoldering MM (SMM). Our aim was to investigate in depth the level of clonal heterogeneity of recurrent genetic abnormalities in these conditions. Experimental Design: Immunoglobulin heavy chain (IGH) translocations, 13q14 and 17p13 deletions, and 1q21 gains using FISH were evaluated in 90 MGUS, 102 high-risk SMM, and 373 MM. To this end, we not only purified plasma cells (PC) for the FISH analysis (purity > 90%), but subsequently, we examined the correlation between the proportion of PC with cytogenetic changes and the number of clonal PC present in the same sample, as measured by multiparametric flow cytometry. Results: We observed a significant difference between the proportion of clonal PC with specific genetic abnormalities in MGUS compared with SMM and in SMM compared with MM. Thus, the median proportion of PC with IGH translocations globally considered, t(11;14) and 13q deletions was significantly lower in MGUS than in SMM, and in SMM than in MM [IGH translocations: 34% vs. 57% vs. 76%; t(11;14): 38% vs. 61% vs. 81%; and 13q deletion: 37% vs. 61% vs. 74% in MGUS, SMM, and MM, respectively]. For t(4;14), the difference was significant in the comparison between MGUS/SMM and MM and for 1q between MGUS and SMM/MM. Conclusions: This study demonstrates that the progression from MGUS to SMM, and eventually to MM, involves a clonal expansion of genetically abnormal PC. Clin Cancer Res; 17(7); 1692–700. ©2011 AACR.