An intestinal permeability test with a combination of 51Cr-EDTA and 14C-mannitol was performed under routine conditions on 176 occasions in 161 adult patients. Of these patients, 116 were under investigation for possible coeliac disease, 33 were known to have coeliac disease, and 12 had inflammatory bowel disease. Small-bowel biopsies were performed in 61 patients. Expressing the results as the ratio of the 6-h urinary recoveries of the two probes was as sensitive as (95%) but more specific for histologic mucosal abnormality (62% versus 46%) than measuring the urinary recovery of 51Cr-EDTA alone. All but two of the patients with active inflammatory bowel disease, whether Crohn's disease or ulcerative colitis, had an abnormal ratio. The 51Cr-EDTA/14C-mannitol intestinal permeability test with a 6-h urine collection is a rapid and simple test of small-intestinal function suitable for routine use.