Colitis was induced with trinitrobenzene-sulphonic acid (TNB) in rats and a histomorphometric study was performed as a possible scoring system for disease activity. The affected colon was examined 10, 20, 40 and 60 days after TNB administration. Quantitative microscopic analysis was performed on the following histologic parameters: necrosis, mucosal epithelium, muscularis fibres, inflammation, granulation tissue and fibrosis. Clinically, the rats were sick, especially on days 10 and 20 after TNB injection. Concomitantly, a peak necrosis score involving the full thickness of the colonic wall was recorded on day 10. The inflammatory reaction was most intense 20 days after TNB injection. After 60 days, marked epithelial regeneration was seen and most of the inflammatory reaction had subsided. A good correlation was found between clinical features and quantitative histomorphometric characteristics of colitis. The criteria described allow a precise description and quantification of the inflammatory and healing process of TNB-induced colitis in rats.