A test of intestinal mucosal function which utilizes the differential permeability of L-rhamnose and lactulose has been reported to be helpful in the diagnosis of gluten-sensitive enteropathy. We have applied this test to 48 male subjects with diabetes mellitus to evaluate its usefulness as a screening test in diabetic patients and to further study sugar absorption in these individuals. Total urinary lactulose excretion in the 13 healthy control subjects was 54.5 +/- 8.5 mg/5 h, while excretion by diabetic patients was increased at 116.1 +/- 15.7 mg/5 h (p less than 0.01). Similarly, total L-rhamnose excretion by diabetic patients was significantly higher (139.7 +/- 14.3 mg/5 h vs 84.3 +/- 18.4 mg/5 h, p less than 0.05). The ratio of percent urinary excretion for lactulose/L-rhamnose (L/R ratio) for diabetic patients (0.197 +/- 0.024) was not different from the control subjects (0.151 +/- 0.2). Nine out of 48 diabetic patients studied had lactulose/L-rhamnose ratios higher than the mean plus two standard deviations of the control group, which might lead to the diagnosis of small bowel mucosal disease. Although we may have been detecting subclinical mucosal disease or gluten sensitive enteropathy in a subgroup, it appears that this test of intestinal mucosal function should be interpreted with caution in diabetic patients.