Epithelial cell proliferation in the duodenum was investigated in 50 patients by incubating mucosal biopsy samples with tritiated thymidine, followed by autoradiography. Fifteen patients had a normal duodenum, 15 duodenal ulcer undergoing elective surgery, 10 perforated duodenal ulcer, and 5 severe non-ulcer-associated duodenitis. The mean crypt cell labelling index in the duodenal bulb of controls was 8.8 +/- 0.4% (mean +/- SEM), at the edge of perforated ulcers 19.1 +/- 2.0%, at the edge of elective ulcers 18.6 +/- 1.4%, and in biopsy specimens from non-ulcer-associated duodenitis 14.0 +/- 1.2%. The mean labelling index in the distal first part of duodenum of control patients was 9.1 +/- 0.8 similar to the values found in histologically normal specimens distal to ulcer or duodenitis. The results indicate active epithelial cell proliferation in both duodenal ulcer and duodenitis. There was no evidence of impairment of epithelial cell proliferation in duodenal ulcer patients.