The pathogenesis of Wegener's granulomatosis, microscopic polyangiitis and idiopathic rapidly progressive glomerulonephritis (RPGN) is still unclear; in vitro data support both humoral and cellular autoimmune mechanisms. An association of Wegener's granulomatosis with HLA antigens has been described, with conflicting results concerning the antigens involved. We have performed serological HLA typing of patients at two different laboratories within the Netherlands (N = 118 and N = 106,N respectively). A significant decrease in the frequency of HLA-DR13DR6 was present in both patient groups in comparison to controls (chi 2 = 21.9; corrected P value < 0.004 for both groups together). There were no differences in the distribution of HLA-antigens between patients with Wegener's granulomatosis and microscopic polyangiitis, between cANCA (cytoplasmic anti-neutrophil cytoplasmic antibodies) and pANCA (perinuclear ANCA, anti-MPO) positive patients, and between patients with and without relapsing disease.