Monoclonal gammopathy of undetermined significance (MGUS) is the most common type of monoclonal gammopathies. Genetic changes, various cytokines and bone marrow angiogenesis play an important role in the pathogenesis. As far as the malignant transformation of MGUS is concerned, size and type of the serum M-protein, serum kappa and lambda free light chain ratio and number of plasma cells in peripheral blood seem to play a predictive role. A new possible risk-stratification model predicting progression of MGUS to multiple myeloma or other related disorders was presented in 2006. The model takes three parametres in consideration, type and initial size of the serum M-protein and serum kappa and lambda ratio. Patients are divided into four risk groups with different risk of progression, from 5% at 20 years in low risk group to 58% in high risk group. The interval from MGUS diagnosis to the evolution of multiple myeloma or other related malignancies ranges from 1 to 30 years. Nevertheless, the risk of progression persists even after more than 30 years after MGUS diagnosis.