The lactulose hydrogen breath-test provides a non-invasive measure of mouth to caecum transit-time. Its use, particularly in children has not been thoroughly investigated. We have studied the effects on transit time of altering the lactulose concentration of the test solution, and of administering it in a liquid nutrient preparation. Concentration markedly affected transit-time; studies with an isotonic solution may be least affected by delayed gastric emptying, and so may reflect small bowel transit-time. Day-to-day variation was large, and was not reduced by altering lactulose concentration, or by using the liquid nutrient based test solution. There was no correlation between age and transit-time.