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Background: Lactose intolerance with adult-onset is due to the inadequate enzymatic activity of lactase-phlorizin hydrolase (LPH). It is frequently seen in patients with Crohn disease, but the mechanism remains to be elucidated. Two DNA genotypes, C/C -13910 and G/G -22018 , located upstream from the LCT locus, the gene encoding for LPH, were recently identified as representing genetic markers for lactose intolerance. We utilized these two DNA genotypes to study their role in inflammatory bowel disease. Methods: We investigated the prevalence of these two DNA variants using specific restriction enzyme digest assays in 166 patients with Crohn disease, in 120 healthy first-degree relatives of Crohn disease patients, in 63 patients with ulcerative colitis and in 187 healthy individuals. Results: The analysis revealed a frequency of 21.4% of the 2 genotypes for adult-type hypolactasia in our studied German cohort of healthy individuals, which is higher than previously reported (15%) based on the hydrogen (H 2 ) breath test. This might indicate a higher sensitivity of genotyping, but it has to be confirmed in larger cohorts. No significant difference was detectable in the frequency of the C/C -13910 and G/G -22018 genotypes in patients with Crohn disease (C/C -13910 : 21.7%; G/G -22018 : 22.3%) compared to first-degree relatives (C/C -13910 : 21.7%; G/G -22018 : 20.8%), patients with ulcerative colitis (C/C -13910 : 20.3%; G/G -22018 : 20.3%) and healthy individuals (C/C -13910 : 21.4%; G/G -22018 : 21.4%). Conclusion: The C/C -13910 and G/G -22018 genotype of adult-type hypolactasia is not associated with susceptibility to the pathogenesis of Crohn disease and ulcerative colitis. [ABSTRACT FROM AUTHOR]