SUMMARY: A 2-year-old male Boxer dog had passed loose faeces mixed with fresh blood and mucus for 8 months. Tenesmus after defaecation was a feature. Colitis was diagnosed from the proctoscopic appearance of the recto-colonic mucosa and confirmed from a biopsy. The disorder proved unresponsive to sulphasalazine therapy, but oral chloramphenicol, betamethasone and prednisolone enemas administered over a 6-week period produced a satisfactory clinical improvement, which persisted for a further 3 weeks without treatment. However, follow-up proctoscopy showed only a marginal improvement in the appearance of the mucosa and appeared to exacerbate further bloody diarrhoea, which persisted. The dog was destroyed and histiocytic ulcerative colitis confirmed at autopsy.