ANCA are associated with certain forms of systemic vasculitis, and have been reported previously to be of the IgG and IgM isotype. We examined the possible association between IgA ANCA and the IgA-related diseases Henoch-Schönlein purpura (HSP) and IgA nephropathy (IgAN). IgA and IgG ANCA were detected by isotype-specific solid-phase assays with a crude neutrophil extract, and their presence was confirmed by antigen-specific fluid-phase competitive inhibition tests and by indirect immunofluorescence. The possible interference by IgA rheumatoid factor was excluded. IgA ANCA were detected in sera from 11/14 HSP patients (79%), from 1/30 IgAN patients (3%), from 1/40 patients with vasculitides classically associated with IgG ANCA (2.5%), and in none of 60 sera from healthy blood donors. IgG ANCA were present with IgA ANCA in three patients with HSP. Only one HSP serum had anti-myeloperoxidase (MPO) activity by both IgA and IgG isotype-specific ELISA, and none was positive for proteinase 3 (PR3). Western blot analysis performed with neutrophil extract showed that the four strongest IgA ANCA-positive HSP sera reacted with a 51-kD protein; Western blot performed on cellular fractions showed that this protein is primarily membrane-associated, and different from fibronectin. Our study suggests that adult HSP is closely associated with circulating IgA ANCA, which may be directed against a different autoantigen than that recognized by IgG ANCA.