A 20-year health follow-up study of 159 initially healthy blood donors with a severe deficiency of serum IgA ( < 0.05 x 10(-3) g/L) and of 45 donors with decreased serum IgA (0.05 x 10(-3)-0.8 g/L) was carried out. The findings indicate that persons with a severe deficiency of and decreased serum IgA who are healthy as young adults have an increased susceptibility to pneumonia and recurrent episodes of other respiratory infections and a higher risk of developing autoimmune diseases in middle age. Vitiligo, autoimmune hypothyreosis, milk intolerance, and possible rheumatoid arthritis were associated with severe IgA deficiency, but otherwise different degrees of IgA deficiency seem to be similar with respect to the appearance of diseases. Regardless of the fact that a total of 163 (80%) of the 204 IgA-deficient subjects had-episodes of infections, drug allergy, or autoimmune or atopic disease, the finding of primary, selective IgA deficiency in a healthy adult per se does not seem to predict severe life-threatening illnesses at least during 20 years of life.