Eighty-seven patients with monoclonal gammopathy of undetermined significance (MGUS) were followed for a period of 1-20 years, median 91 months. Transformation to multiple myeloma occurred in 14 patients of whom seven died as a consequence of the disease. There were 13 unrelated deaths. The actuarial probability of survival was 80% at 10 years and 44% at 15 years and the probabilities of malignant conversion for the same periods were 17% and 30% respectively. The most significant factor influencing the probability of malignant conversion was the increase of monoclonal protein above the level of 30 g/l during the observation period (P<0.001), followed by an increase of M-protein to more than 50% above the baseline level (P=0.02) and a decreased level of uninvolved immunoglobulins (P=0.054).