INTRODUCTION: There are inflammatory bowel disease (IBD) patients avoid lacteal products without evidence of lactose malabsorption, probably because of incorrect patient perceptions and arbitrary advice from physicians and diet books. AIM: To evaluate the prevalence of lactose malabsorption in patients with IBP. METHOD: In 18 patients with Crohńs disease and 24 with ulcerative colitis and 25 controls the prevalence of lactose intolerance, as measured by lactose breath hydrogen tests. RESULTS: Observed deficiencies of absorption of the lactose in 7 (16.6%) patients with IBP and 5 (20%) subjects control. CONCLUSIONS: Not all the patients with IBD are intolerant to the lactose by the suppression of the lacteal produts must not be generalized in the diet of these patients.