18 control subjects and 18 patients, with a variety of gastrointestinal conditions were investigated using a 10-muCi 14C-D-xylose breath test. The latter also underwent quantitative bacterial studies of fluid obtained by intestinal intubation. In 14 patients a smaller dose of 3 muCi 14C-D-xylose was compared to the standard dose and there was a good correlation between the two doses. The peak value of the 14C-D-xylose test provided the best discrimination between patients with and without bacterial overgrowth. The 14C-glycocholic acid test performed in 15 patients, although as sensitive, was less discriminating. The 14C-D-xylose breath test is reliable and more specific in confirming the diagnosis of small intestinal bacterial overgrowth without having to resort to direct bacterial studies.