The accuracy of the 14C-D-xylose breath test in the diagnosis of small-bowel bacterial overgrowth was prospectively evaluated in 10 patients with motility disorders: 6 myopathic, 3 neuropathic, and 1 mechanical obstruction. Six of the 10 patients had small-bowel bacterial overgrowth (> or = 10(5) colony-forming units/ml) on culture of small-bowel aspirate. Increased breath 14CO2 levels were documented in three of six patients with positive cultures and in two of four with negative cultures. Two patients with positive results by both methods and one of two patients with positive breath 14CO2 but negative cultures had previously undergone gastric surgery. Three patients with myopathic dysmotility had positive cultures but negative breath tests. Cultures of duodenal aspirates and the D-xylose test had sensitivities of 80% and 40%, respectively, for the finding of hypoalbuminemia. Compared with cultures, the sensitivity and specificity of the breath test were 60% and 40%, respectively. Impaired delivery of 14C-D-xylose for bacterial metabolism may result from postprandial antral hypomotility (n = 4) or low-amplitude (n = 6) small-bowel motility, contributing to the false-negative breath tests. Thus, culture is the optimal method to detect small-bowel bacterial overgrowth in patients with motility disorders.