Sera that had been positive in routine ELISA for ANCA were studied retrospectively for the IgG subclass distribution of these autoantibodies. An ELISA previously developed for measurement of IgG subclasses of anti-GBM antibodies was modified for this purpose. Of a total of 247 sera, 114 were found to be positive in at least one of the assays for IgG subclasses of anti proteinase 3, 72 of these patients were men and 42 were women, giving a ratio of 1.8. Also 134 sera were positive in at least one of the IgG subclass assays for antimyeloperoxidase (MPO), with a male/female ratio of 0.97. The ANCA seem to consist mainly of IgG1 and IgG4 autoantibodies. Among the anti-MPO group, IgG2 is relatively common and IgG3 is scarce. Contrasting with this, IgG3 is relatively common in the antiproteinase 3 group. In this group high IgG2 titres are rare. Twelve sera were found to be positive for both autoantigens. Clinical data were studied for 44 patients. Prognosis for old patients was found to be poor. Patients with inactive disease were often positive in only one subclass assay, while patients with active disease were positive in two or more subclass assays (P < 0.01).