The records of 519 patients with a paraproteinaemia who where examined at our institutions before January, 1, 1983, were reviewed. After patients with multiple myeloma, macroglobulinemia, amyloidosis, lymphoma or related diseases had been excluded, 363 cases remained which fulfilled the criteria for a diagnosis of idiopathic monoclonal gammopathy (IMG). Also excluded were the cases in which follow-up efforts failed and those in which the paraprotein had disappeared. A long-term study was thus possible in 313 patients with IMG. They were arbitrarily subdivided into two groups: 213 patients with a follow-up of 5-8 years and 100 patients whose follow-up was longer than 8 years. Fourteen per cent of the patients from the former group and 18 per cent from the latter developed a malignant B cell dyscrasia, on the average this event becoming clinically detectable after 63 months (27-138 months) from initial recognition of the paraprotein. A retrospective analysis of the clinical data at the diagnosis of IMG, revealed that none of the patients could have been identified in advance in whom a malignant transformation later occurred.