In a previous study, we reported that the high mobility group (HMG) non-histone chromosomal proteins HMG1 and HMG2 were novel target antigens of P-ANCA. In this study, we determined the immunodiagnostic value of anti-HMG1/HMG2 antibodies in patients with UC. Sixty sera from patients with UC were tested for reactivity with HMG1 and HMG2 by means of ELISA. Anti-HMG1 antibody was detected in 32% of patients (40% of P-ANCA+ patients). Anti-HMG2 antibody was detected in 33% (40% of P-ANCA+ patients). Thirty-five percent of sera were positive for antibody to either HMG1 or HMG2 (43% of P-ANCA+ patients). P-ANCA+ patients expressed anti-HMG1/HMG2 antibodies with significantly greater frequency compared with P-ANCA- patients. Furthermore, the anti-HMG1/HMG2 antibodies were significantly related to disease activity in UC. Sixteen of the 18 UC patients, who had high titres of anti-HMG1 or -HMG2 antibody during the active phase, showed lower titres in the inactive phase. Anti-HMG1/HMG2 antibodies appear to be useful as a marker for disease activity in UC.