Samples of ileal serosa and mesenteric lymph nodes have been harvested before antibiotic administration during 46 non-contaminated operations for Crohn's disease and compared with 43 operations for conditions other than Crohn's. Potentially pathogenic bacteria were isolated from the serosa in 12 (27 per cent) Crohn's patients, compared with 6 (15 per cent) controls (P = 0.04). Intestinal bacteria were recovered from mesenteric nodes in 15 (33 per cent) Crohn's patients compared with 2 (5 per cent) controls (P = 0.006). These findings suggest that bacteria leak from the small bowel lumen in a high proportion of Crohn's disease patients. This may explain the pathogenesis of abscess and fistula in this disorder as well as the high rate of sepsis following elective surgery even in the absence of macroscopic contamination.