The aim of this study was to determine the nationwide incidence of collagenous and lymphocytic colitis in Iceland and the location of histopathological changes in the large bowel. All pathology reports of patients diagnosed with or suspected of having collagenous colitis or lymphocytic colitis in the period 1995-1999 were identified. All pathology samples were reevaluated using strict diagnostic criteria. After reevaluation 125 patients fulfilled our diagnostic criteria, 71 as collagenous colitis and 54 as lymphocytic colitis. The mean annual incidence for collagenous colitis was 5.2/100,000 inhabitants, and the mean age at diagnosis was 66.1 years. The mean annual incidence for lymphocytic colitis was 4.0/100,000 inhabitants, the mean age at diagnosis was 68.7 years. Both diseases more commonly involved the colon than the rectum. The incidence of collagenous colitis and lymphocytic colitis is high in Iceland. The mean annual incidence of collagenous colitis is much higher in Iceland than hitherto reported elsewhere.