Bactericidal/permeability-increasing protein (BPI) and azurocidin (AZ) are recently described target antigens of antineutrophil cytoplasmic antibodies (ANCA). In this study, BPI-ANCA were demonstrated most often in patients with ulcerative colitis (36/92, 39%), Crohn's disease (17/66, 26%) and cystic fibrosis (11/14, 79%), but also in patients with rheumatoid arthritis (8/40, 20%), systemic lupus erythematosus (SLE) (111/65, 17%) and mixed connective tissue disease (4/18, 22%). BPI-ANCA were also common in sera containing antinuclear (ANA) (9/43, 21%) or antidouble-stranded (ds) DNA (7/28, 25%) antibodies. There was no increased frequency of abnormal alpha1-antitrypsin (alphal1AT) phenotypes in patients with BPI-ANCA, and BPI-ANCA were not more common in individuals with an abnormal phenotype. The predominant IgG subclasses were IgG1 and IgG3; IgA but not IgM was present. Both IgG and IgA BPI-ANCA were high affinity antibodies, and the affinity of IgG antibodies did not change with time in the sera tested. Four of the five sera (80%) containing BPI-ANCA did not bind to denatured, reduced BPI, suggesting that most BPI-ANCA recognised conformational epitopes. AZ-ANCA were demonstrated in 2/11 patients (18%) with Wegener's granulomatosis, 3/12 (25%) with cystic fibrosis and 3/14 (21%) with chronic active hepatitis. AZ-ANCA were present in 5/25 sera (25%) with ANA, but the levels were only marginally elevated. AZ-ANCA were uncommon in patients with inflammatory bowel and rheumatological diseases, and in sera containing other autoantibodies. Again, there was no association with abnormal alpha1-AT phenotypes. BPI represents a major ANCA target antigen in patients with rheumatological as well as inflammatory bowel disease and cystic fibrosis, but AZ-ANCA are uncommon.