Fifty-three patients with bronchiectasis (chronic bronchial suppuration) of unknown cause, in whom the chronic production of purulent sputum was the prominent clinical feature, were investigated for possible immunological abnormalities. They were compared with two control groups comprising 50 patients with chronic bronchitis and emphysema and 33 patients with bronchial asthma. Forty-two patients with bronchiectasis(79%) had at least one abnormality of immunoglobulin, usually elevation of IgA, IgG or IgM. Eight patients had all three immunoglobulin levels raised and this was related to severity of disease. Similar increases in immunoglobulin levels were observed in the control groups, but the frequency and severity of these changes were significantly greater in the bronchiectasis patients. Two patients had IgA deficiency. There was a very high prevalence of rheumatoid factor (52%) and an increased prevalence of antinuclear factor (10%) in the bronchiectasis patients compared with the control groups. The presence of these autoantibodies did not correlate closely with severity of disease. Ten patients with bronchiectasis (19%) had one or more autoimmune disorders, and the association of severe bronchiectasis, Hashimoto's thyroiditis and pernicious anaemia in one patient is described in detail. The immunoglobulin changes, high incidence of autoantibodies and association with autoimmune disorders raises the possibility that in some patients with bronchiectasis (chronic bronchial suppuration) of apparent unknown cause abnormal immune mechanisms may be important in causing or perpetuating the condition.