Twelve patients with skin vasculitis complicating cystic fibrosis are described. Seven of these were proven histologically and of these two had systemic vascultitis. Staining of vasculitic tissue by the avidin-biotin immunoperoxidase technique using both monoclonal and polyclonal antisera directed against Haemophilus influenzae, staphylococcus aureus and Pseudomonas aeruginosa did not consistently reveal any bacterial antigens in these tissues. In one patient the vasculitis appeared secondary to ranitidine. There was no evidence of autoimmune disease in any of the patients. Antineutrophil cytoplasmic antibodies were detected in the serum of 40 per cent of the patients with vasculitis complicating cystic fibrosis but in none of 61 controls with cystic fibrosis (but without vasculitis) matched for age and sex and with similar bacteriological flora of sputum.