HLA supratypes as well as serum IgG, IgA and IgM levels were determined in 44 children and adolescents with severe IgA deficiency (serum IgA less than 5 mg/dl) and in first degree relatives. Frequencies of the HLA alleles B14, DR1, DQW1, C4A2 and C4B2 were significantly higher in the IgA-deficient patients than in the controls. The most recurrent haplotype among patients was B14, DR1 (p less than 10(-4) preferentially associated with A33, A28 or A blank. The supratype B14, Bfs, C4A2, C4B2, DR1, DQW1 was present in a 14-fold higher frequency than in the controls, and strongly suggests the presence of a gene in the HLA region involved in the deficiency of IgA. The fact that this supratype was not always associated with IgA deficiency in the parents and that not all IgA-deficient subjects had this supratype is discussed. Severe IgA deficiency was found in four mothers (two of the four mother-child pairs shared the B14, DR1 haplotype); three other relatives (one father and two brothers) had partial IgA deficiency and did not have the B14, DR1, DQW1 haplotype.