Breath-hydrogen production after oral glucose administration was examined in patients suspected of having small-intestinal colonisation and compared with the 14C-glycine-cholate breath test (14C-G.C.) and with bacteriological examination of the small intestine. Of 17 patients, 12 had bacteriological evidence of small-intestinal colonisation. Each breath test showed 8 of the 12 patients to be colonised, but only 5 patients gave positive results with both tests. Nevertheless, using both tests only 1 patients out of 12 with small-intestinal colonisation would have been missed. There were no false-positive results in the 5 bacteriologically normal patients when the breath-hydrogen test was used. It is concluded that simultaneous use of these two relatively simple breath tests may improve the indirect diagnosis of small-intestinal colonisation.