There is an increasing prevalence of ulcerative colitis in Hong Kong and cytomegalovirus infection is an important factor in the exacerbation of the disease. We report on a 33-year-old Chinese man with ulcerative colitis in remission, who presented with bloody diarrhoea that failed to respond to an intensive regimen of oral and rectal steroid. Colonoscopy was performed and biopsy specimens showed signs of cytomegalic colitis in association with ulcerative colitis. Administration of ganciclovir and the gradual termination of steroid treatment resulted in remission of the colitis. The clinical course suggested an exacerbation of ulcerative colitis due to cytomegalovirus infection. The relationship between ulcerative colitis and cytomegalovirus is discussed.