Background: Disease activity can be determined using clinical, endoscopic or histologic criteria in patients with ulcerative colitis (UC). Persistent disease activity is associated with poor outcomes. Histologic disease activity has been shown to be associated with relapse, colectomy and colorectal cancer. The ability to objectively evaluate microscopic disease activity using histology is important for both clinical practice and clinical trials. However, the operating properties of the currently available histologic indices remain unclear.