The obligate anaerobic faecal floras of patients with Crohn's disease, their first-degree relatives, and healthy control subjects were compared. The flora of Crohn's patients contained more anaerobic gram-positive coccoid rods and gram-negative rods than that of healthy subjects; on this basis patients and healthy subjects formed two clusters with minor overlap. Nine of 26 children of Crohn's patients were also included within the Crohn's disease cluster. During 5 to 7 years of follow-up study 3 of them presented with remitting abdominal pain, diarrhoea, or weight loss, and in 1 of them Crohn's disease was diagnosed; none of the 17 children with a normal flora showed symptoms possibly due to Crohn's disease. It is concluded that the abnormal flora may be indigenous to subjects predisposed to Crohn's disease, suggesting a direct or indirect relationship between the abnormal faecal flora and Crohn's disease.