The diagnostic criteria, incidence, clinical characteristics and outcome of 397 patients with monoclonal gammopathies of undetermined significance, all of them diagnosed and followed-up at the Haematology Service of the Miguel Servet Hospital, in Zaragoza, between January 1970 and December 1988, were revised. The patients' mean age was 64.7 years (range: 2-89). The M/F ratio was 236/161. The mean concentration of the M component (MC) was 1.17 g/dL (range: 0.20-3.50), this being under 0.50 in 65 cases. IgG was the most frequent MC (71.26%), followed by IgA (14.34%) and IgM (10.82%). Multiple MC was present in 14 cases (3.58). Light chains were passed in urine by 33 patients (8.31%). No associated pathology was found in 213 patients (53.65%) upon MC discovery, while 65 other (16.31%) were carriers of different blood disorders, chronic lymphoproliferative diseases being the commonest (11.57%). In 30 patients (7.30%) the MC was associated to nonhaematological malignancies, and 29 others had an underlying chronic infection. Chronic liver disease was present in 25 cases, and autoimmune disease in 14. Transient monoclonal gammopathy was seen in a small group of patients (6.54%), most of them suffering from acute infectious illness. With regard to the group of patients without any associated pathology, their median follow-up was 37.8 months (range: 18-228). Of them, the MC kept unchanged in 134 cases (62.91%); 47(22.06%) died from any unrelated cause, and 10 others evolved into malignant monoclonal gammopathy. The median clinical course of these last expanded to 60 months (range: 11-124), with an accumulated actuarial risk of 4.5% at 5 years, 15% at 10 years and 26% at 15 years.