Summary. The term monoclonal gammopathy (MG) signifies the benign or malignant clonal growth of B lymphocytes. In the present study, monoclonal gammopathy of unknown significance (MGUS) was defined as those patients with no identified haematological malignancy. A database was constructed of all 713 MG patients in Iceland between 1976 and 1997 and compared with the Icelandic Cancer Registry. The age-standardized incidence per 100 000 of MG was 10·3 for males and 8·6 for females, calculated for the whole period, rising steadily from 5·8 (men) and 4·9 (women) during the 5-year period 1976–80 to 14·7 (men) and 12·5 (women) during the last 5 year period. Age-standardized incidence rates were very low for subjects under 50 years of age, then increased with age from 11 and 17 per 100 000 at 50-54, to 169 and 119 per 100 000 at age 80–84, for men and women respectively. No association was detected between MG and non-haematological malignancies, neither retrospectively nor prospectively. Haematological malignancy was diagnosed in 209 (29·3%) cases before the recorded finding of MG or within the same calendar year, leaving 504 (70·7%) patients diagnosed with MGUS. Of these, 51 (10%) progressed to multiple myeloma or Waldenström's macroglobulinaemia after a mean interval of 3·8 years; mean follow-up was 7·4 years, median 6 years. The most common immunoglobulin (Ig) class was IgG (55%), followed by IgM (32%) and IgA (13%). MGUS was a highly significant risk factor for developing haematological malignancies and the risk was significantly greater for MG of the IgA class compared with either IgG or IgM.