Bacterial overgrowth is frequent in patients with Crohn's disease (CD) and can contribute to symptoms. Motility abnormalities can predispose to bacterial overgrowth. The hydrogen (H2) and methane (CH4) breath test is a sensitive and simple tool for the diagnosis of bacterial overgrowth and for the evaluation of orocecal transit time (OCTT). In this study, we investigated the prevalence of OCTT modifications and bacterial overgrowth in a series of consecutive adult patients with CD. In 43 healthy subjects and 67 patients with CD, we performed the lactulose breath test using a gas analyzer that offers the opportunity of measuring both H2 and CH4. Of the patients, 24 had undergone an ileocolic resection before the test with ablation of the ileocecal valve. At the time of the test 15 patients had active disease, whereas in 52 subjects the disease was quiescent. Fifty-seven patients and forty controls were evaluable for OCTT and bacterial overgrowth. In 10 patients and in 3 controls, no H2 or CH4 peak was recorded during the 8-hour test. Out of 57 patients, 13 (23%) were affected by bacterial over-growth. The prevalence of bacterial overgrowth was higher in patients with previous surgery (30%) than in nonoperated patients (18%). In all patients with bacterial overgrowth, an antibiotic treatment induced a normalization of the test and an improvement of the symptoms. We observed a longer OCTT in the patients compared to controls, although this difference was not statistically significant (154 ± 45 vs. 136 ± 45 minutes). OCTT was significantly longer compared to controls in the 14 CD patients with previous ileocolic resection (180 ± 53 vs. 136 + 45 minutes; p < 0.004). In conclusion, we found that a significant proportion of unselected patients with CD has bacterial overgrowth and prolongation of OCTT. We suggest that the modifications in OCTT in patients with CD can predispose to bacterial overgrowth. The lactulose breath test is a simple method that can be more widely used in patients with CD.