Disclosed is a method of detecting small intestinal bacterial overgrowth (SIBO) in a human subject. The method involves detecting the relative amounts of methane, hydrogen, and at least one sulfur-containing gas in a gas mixture exhaled by said human subject, after said human subject has ingested a controlled quantity of a substrate, said gas mixture being at least partially produced by the intestinal microflora of said human subject. Kits for use in such detection methods are also disclosed.