The actuarial probability of malignant transformation and the impact on expected survival were analysed in a series of 128 persons diagnosed with monoclonal gammopathy of undetermined significance (MGUS) over a 20-year period. At a median follow-up of 56 months the M-component remains stable in 101 patients (78.9%), 14 patients (10.9%) have died from non-related disorders and 13 (10.2%) have developed malignant transformation of MGUS (multiple myeloma, 10; primary amyloidosis, two; Waldenström's macroglobulinaemia, one). The actuarial probability of malignant transformation at 5 and 10 years was 8.5% and 19.2%, respectively. When different presenting features were analysed for predictive value of the malignant transformation, the IgA type of MGUS was the only variable associated with a higher probability of such an event (P less than 0.025). Although no significant difference was observed between the survival probability of persons with MGUS and that of the control population, the development of malignant transformation was associated with a shorter survival (P less than 0.001).