AIMS--To study the effect of proctocolectomy on the antineutrophil cytoplasmic antibody (ANCA) titres in association with ulcerative colitis. METHODS--Serum samples were taken from 15 patients with ulcerative colitis immediately before and at a mean of 24 months after proctocolectomy. Indirect immunofluorescence for ANCA and enzyme immunoassays for myeloperoxidase and proteinase-3 antibodies were employed. A liver biopsy was taken from every patient during the proctocolectomy, and serum liver enzyme activities were also determined. RESULTS--Before proctocolectomy, 13 of the 15 patients had perinuclear antineutrophil cytoplasmic antibodies (p-ANCA). Additionally, one patient had a low tire of classical cytoplasmic ANCA and one had granulocyte specific antinuclear antibodies. After proctocolectomy, the ANCA titres decreased in 10 patients, in two of whom they became negative. The titres remained the same in four patients with positive ANCA and increased twofold in one patient. Only one patient was proteinase-3 antibody positive and all 15 patients were myeloperoxidase antibody negative. The clinical condition improved in all patients, irrespective of the ANCA status after proctocolectomy. Seven patients, all of whom were positive for p-ANCA before proctocolectomy, had histological liver abnormalities. No correlation was observed between serum liver enzyme levels and ANCA staining patterns or titres. CONCLUSIONS--Proctocolectomy decreased the ANCA titres in the majority of our patients, suggesting that reduction of the inflammation or the available antigenic material modifies the immune disturbance related to ulcerative colitis.