The objectives of the present study were to compare the interleukin 6 (IL-6) and tumour necrosis factor-alpha (TNF-alpha) serum levels of individuals with monoclonal gammopathy of undetermined significance (MGUS) with those of healthy controls, and to ascertain the predictor value of these cytokines in the evolution from MGUS to multiple myeloma. After a median follow-up of 7 years from the initial cytokine measurements, nine patients with MGUS have evolved to a malignant condition. The actuarial probability of malignant transformation in patients with increased IL-6 and TNF-alpha was not significantly higher than in those with normal values.