Fifty children with IgA deficiency were folllowed for 1 to 4 years from 1975 to 1978. Thirty-five had complete deficiency of serum IgA (less than 2.5 IU/ml) and 15 partial deficiency (serum IgA below the 10th centile for age). Patients with another associated immunodeficiency, such as ataxia-telangiectasia, were not included. Most children with complete deficiency of IgA had recurrent respiratory and/or gastrointestinal infections, about half with onset in the first year of life, while partial deficiency of IgA has probably little if any importance for anti-infectious immunity but is important in the pathogenesis of atopy. Atopic diseases were frequent in both groups. Chromosomal abnormalities were found in 2 patients: trisomy 21 in one and in the other a ring chromosome 18. No important defects in cellular immunity were detected but some isolated, borderline abnormalities were often present.