Prospective studies on the risk of malignant transformation in patients with monoclonal gammopathy of undetermined significance (MGUS) and factors predictive of survival are lacking. The Dutch Comprehensive Cancer Centre West, comprising 1.6 million inhabitants, initiated a prospective hospital-based cohort study on 1464 patients with newly diagnosed M-proteinaemia, median age 73 (17-103) years. M-protein related diagnoses, patients' characteristics, laboratory investigations, bone marrow examinations and skeletal X-rays were registered with a yearly follow-up. Main endpoints were death, or new diagnoses of multiple myeloma and non-Hodgkin lymphoma. Kaplan-Meier survival curves were compared with age- and gender-matched survival data from the total Dutch population. Cumulative malignant transformation was corrected for death using a competing risk model. Risk factors for transformation or death were analyzed by univariate and multivariate analyses. In 1007 MGUS-patients, malignant transformation was associated with rising M-protein levels, IgA and IgM isotype and occurred at a yearly rate of 0.4%. All MGUS patients survived less than a matched cohort of the Dutch population, even in the absence of M-protein-associated comorbidity. Serum albumin levels at entry appeared highly predictive for survival. M-proteinaemia is not an innocent symptom. Although malignant transformation occurs rarely, survival is shortened irrespective of comorbidity.