Intestinal permeability has been assessed as a parameter of the small intestinal barrier function in 33 patients with AIDS (WR 6), in 25 healthy volunteers, 32 patients with Crohn's disease, 18 patients with ulcerative colitis and in 12 patients with untreated coeliac disease. Permeability was measured by means of the 5 hour urinary excretion of lactulose and rhamnose after administration of these sugars (1 g rhamnose, 10 g lactulose) which allowed to calculate the urinary lactulose/rhamnose--ratio (L/R-r) as an index of intestinal permeability. Compared to the controls (L/R-r = 0.014 +/- 0.010) patients with AIDS had significantly increased lactulose/rhamnose--ratios (0.189 +/- 0.164; p less than 0.01) indicating abnormal permeation both of lactulose and rhamnose. This alteration of intestinal permeability was more pronounced than the significant increase of the L/R-r in the patients with either Crohn's disease or coeliac disease. However, intestinal permeability was not altered in the patients with ulcerative colitis. Among the patients with AIDS, the most abnormal permeability ratios were observed in two subjects with intestinal cryptosporidiosis. This investigation demonstrates by means of the "double sugar ratio", that intestinal permeability in patients with AIDS is highly abnormal, the impairment being even more abnormal than in patients with other small intestinal disease.