Patients with a monoclonal gammapathy of undetermined significance (MGUS) are usually submitted to a periodical clinical follow-up, but it is not known if this surveillance can ameliorate the prognosis of a plasma cell malignancy that will be eventually detected. We compared the clinical and laboratory characteristics at onset, the response to chemotherapy and the survival, of 21 cases of newly diagnosed multiple myeloma (MM) arising from the malignant transformation of MGUS and 41 cases without a previous history of MGUS, recruited to the same first-line treatments over a 3-years period. The former group showed a significant lower frequency of advanced stages as well as other several prognostic factors of high risk including anemia, renal failure, bone lesions and increase of beta2 microglobulin and C-reactive protein levels. Despite a similar response to treatment of the two groups, MM arising from MGUS showed a significantly longer median survival than MM without prior MGUS. This was particularly true for stage I, while stages II and III behaved similarly. We conclude that the regular clinical monitoring of MGUS patients allowed the identification of earlier malignant transformation, when tumor burden is lower, as indicated by lower beta2 microglobulin levels and marrow plasmacytosis of stage I MM arising from MGUS. Moreover, a slower proliferation rate of myeloma cells, as suggested by lower C-reactive protein levels, may be considered so as to explain the longer survival of these patients.