The prevalence of human intestinal spirochetosis (HIS) was determined histologically in 402 subjects from South Norway (Kristiansand) who had undergone colorectal resections during the period June 1991-June 1996. The figures were compared with those from a previous mid-Norwegian study (Trondheim) comprising 1205 patients with large bowel resections as well as colonoscopic mucosal biopsies examined in 1990. The studies showed a prevalence of HIS of 2.5% in mid-Norway and 3.0% in South Norway, and in both regions there was a considerable male predominance of HIS (70% and 75%, respectively). Furthermore, HIS was more often diagnosed by the pathologists in South Norway (25%) than in mid-Norway (6.5%).