Bacterial overgrowth of the small bowel has been demonstrated in the blind loop syndrome and implicated in various postgastrectomy states. Vagotomy and antrectomy was performed in 20 patients with duodenal ulcer requiring elective surgery, gastroduodenostomy reconstruction being used in 16 patients and gastrojejunostomy in four patients. Quantitative aerobic and anaerobic bacteriology of the jejunum demonstrated a consistent and marked increase in the number of microorganisms comprising the microbial flora. Correlation of basal and stimulated gastric secretory output with postoperative bacterial overgrowth failed to demonstrate any definite control exerted by gastric acid over bacterial proliferation in the jejunum. The presence of large numbers of microorganisms in the small bowel might relate to several of the postgastrectomy syndromes.