Presence of monoclonal immunoglobulin in serum or urine is a relatively common event affecting about 3.2 % of people over 50. Isolated increase of only one type of free light chain, either κ or λ, is detected in 0.7-0.8 % of people over 50. Most people with monoclonal immunoglobulin meet the criteria of the so-called "mono-clonal gammopathy of undetermined significance (MGUS)". MGUS is defined by concentration of monoclonal immunoglobulin in serum < 30 g/l, number of plasma cells in the bone marrow < 10 % and the absence of symptoms of multiple myeloma and other lymphoproliferative diseases. A proportion of people with MGUS gradually progresses from asymptomatic into symptomatic myeloma or other malignant lymphoproliferative disease requiring treatment. Therefore, MGUS is considered to be one of the most common premalignant conditions with an average risk of transformation into malignant disease of 1 % per year. Monoclonal gammopathy of IgG and IgA subtype can develop into multiple myeloma. Light chain monoclonal gammopathy can develop not only into light chain multiple myeloma but also into AL-amyloidosis and light chain deposition disease (amorphous deposits of light chains damaging organs). IgM monoclonal gammopathy may develop into Waldenstrom macroglobulinemia or other lymphoproliferative disorder, or into rare IgM subtype of multiple myeloma. Unfortunately, people with MGUS are threatened by more than an increased risk of transformation into multiple myeloma or other severe hematologic disease. Pre-malignant clone of plasma cells in the bone marrow causes changes in the bone marrow that directly affect the person. For people with MGUS, there is an increased incidence of osteoporosis and increased fracture risk when compared to the general population. People with MGUS also have an increased risk of bacterial infections and thromboembolic complications compared with the same age population without MGUS. Clonal plasma cells, which are the basis of MGUS, may in some cases produce toxic monoclonal immunoglobulin which can damage the body's own antibody activity by binding to specific antigens (such as cold agglutinin disease), or their deposits in organs (e.g. kidney damage) or physical properties (e.g. cryoglobulinemia). Therefore, it is recommended that this group of people is regularly checked with the aim to capture not only transformation into symptomatic multiple myeloma or another malignant disease, but also the formation of the above-mentioned complications. Moreover, it is recommended to monitor patients with asymptomatic myeloma and to initiate treatment only after symptoms of multiple myeloma are observed. In 2014, discussion of subdivision of subgroups of patients with asymptomatic myeloma with high ( 80 %) probability of early (within 2 years) transformation in multiple myeloma which would be beneficial for early initiation of treatment is ongoing. According to first proposals, patients with asymptomatic myeloma that meet at least one of the three conditions: more than 60 % of plasma cells in the bone marrow, ratio of free light kappa and lambda chains is greater than 100 or less than 0.01, or multiple focal lesions on whole-body MRI of the skelet. The review contains current opinions on prognostic classification and appropriate intervals and extent of control examinations.