An increased intestinal permeability has been proposed as an aetiologic factor in Crohn's disease. The 24-h urinary excretion of 100 muCi 51Cr-labelled ethylenediaminetetraacetic acid (EDTA) was used to test the permeability in 15 patients with Crohn's disease and in 20 healthy first-degree relatives, who are known to have a genetic predisposition to inflammatory bowel disease. Twenty-eight healthy persons not related to patients with inflammatory bowel disease served as control material. The 51Cr-EDTA excretion of the relatives (range, 0.98%-3.87%; median, 1.935%) was not significantly higher than that of the controls (range, 1.17%-3.26%; median, 1.950%), whereas patients with Crohn's disease had a significantly higher excretion (range, 1.64%-8.86%; median, 2.940%) than both the relatives and the controls. Among patients the increased excretion was found only if the small intestine was involved. We conclude that 1) as a group, patients with Crohn's disease in the small intestine have an increased intestinal permeability, in contrast to their healthy relatives, who have a normal permeability; 2) a considerable overlap of the results of the 51Cr-EDTA test was found between the groups studied, and the test is not suitable for evaluating individual patients; 3) our results do not support the hypothesis of an increase in intestinal permeability as an aetiologic factor in Crohn's disease.