Among 1205 mid-Norwegian patients who underwent colorectal biopsies during 1990, 30 cases (2.5%) with human intestinal spirochaetosis (HIS) were found. In only two cases were the spirochaetes mentioned in the biopsy reports. Cases with HIS showed a 7:3 male preponderance, but the mean age did not differ from that in the whole material. The spirochaetes were slightly more frequent in the rectum than in other parts of the large bowel. There was no indication of seasonal or geographical clustering of HIS and no association with occupation, use of certain drugs or underlying diseases. Most cases had HIS as the only positive biopsy finding, but the condition was also seen in connection with adenocarcinomas, Crohn's disease, and metaplastic and granulation polyps. The predominant symptoms of patients with HIS were faecal blood, diarrhoea, abdominal pain and constipation.