The lactulose hydrogen breath test has been evaluated as a diagnostic test for small-bowel bacterial overgrowth using the 14C-glycocholate breath test for comparison. Twenty-seven patients with suspected bacterial overgrowth and 37 control patients were studied. The lactulose test was positive in 8 out of 9 patients with Subsequently proven bacterial overgrowth, all of whom had positive 14C-glycocholate tests. However, 6 patients with ileal disease or resection had positive 14C-glycocholate tests but negative lactulose tests. subsequent bacteriological study of duodenal juice from these patients was negative. Negative results were obtained by both tests in the remaining 12 patients, none of whom were subsequently shown to have bacterial overgrowth. All 37 control subjects had negative lactulose tests. The lactulose breath test is a simple and promising diagnostic test for the detection of small-bowel bacterial overgrowth and, unlike the 14C-glycocholate test, has the advantage of being able to distinguish bacterial overgrowth from ileal disease.