HLA antigens (A, B, C and DR loci) were studied in 62 IgA-deficient (IgAd) paediatric patients: 17 with coeliac disease (CD), 13 with juvenile arthritis (JA), 27 with frequent respiratory tract infections (RTI) and five with other diseases. The frequencies of HLA antigens in IgAd patients were compared with those in healthy blood donors, and in CD and JA patients with normal serum IgA levels. The IgA deficiency in the patients showed significant associations with HLA A1, B8, B13, Cw6, DR3 and DR7 (P less than 0.0005, P corr less than 0.02 vs controls) and decreased frequencies of DR2 (P less than 0.0005, P corr less than 0.02 vs controls). The HLA associations typical of coeliac disease, increased frequencies of HLA-B8 and DR3, were evident among the IgAd coeliacs; in contrast to the coeliacs with normal IgA levels, the IgAd coeliacs showed a significant increase of the HLA-Cw6 allele (P less than 0.0005, P corr less than 0.02 vs control coeliacs). Increased frequencies of HLA-A1, B8, B13, Cw6, DR3 and DR7 were noted in the patients with RTI, which can be explained by the frequent occurrence of the haplotypes A1, B8, DR3 and B13, DR7, the latter haplotype often also having the Cw6 allele. Among the IgAd JA patients, the antigen frequencies were similar to those in the JA patients with normal serum immunoglobulins.