Orocaecal transit time (OCTT) using the lactulose hydrogen breath test was investigated in 16 patients (age 10-19.4 years) with active Crohn disease (CD). Disease activity was assessed by the paediatric CD activity index (PCDAI). OCTT was prolonged in all patients (mean 149.9 min, SD 32.7) relative to healthy age-matched controls (mean 56.9 min, SD 11.1). PCDAI was increased (median 48.8, range 32.5), indicating moderate to severe disease in all patients. A close correlation between OCTT and PCDAI (Spearman r = 0.90) was observed. Following nutritional therapy with a semi-elemental diet over a period of 6 weeks OCTT and PCDAI declined, OCTT becoming normal in 4 and PCDAI in 4/12 patients respectively. OCTT provides a sensitive, non-invasive method for the assessment and followup investigations in patients with CD.