Patients with ulcerative colitis (UC) have a less frequent prior history of appendectomy than the general population. The aim of the present investigation was to elucidate histological and immunological characteristics of the appendix in UC and to assess the effect of appendectomy on the disease. Nine subjects with mildly active UC were treated by surgical appendectomy. In four subjects, the histological findings of the appendix were compatible with ulcerative appendicitis. CD3+CD4+CD25+, CD3+CD4+CD45RO+, and CD3+CD8+CD45RO+ appendiceal mononuclear cells were significantly higher in UC than in acute appendicitis and in normal appendix. There was a trend towards higher mRNA transcripts of IFN-gamma in the appendix of UC than those in other two groups. Clinical activity index decreased significantly four weeks after the appendectomy, although the effect was transient. The appendix is a site of involvement in UC, where mononuclear cells are presumed to be at a state of basal activation.