Abstract: The invention provides a method for diagnosing Inflammatory Bowel Diseases, using genetic markers that are implicated in severe, early-onset Crohn's Disease (CD). The invention also provides coding sequence mutations in the OCTN1 gene that significantly reduces its ability to transport the organic cation carnitine. The invention further provides mutations in the promoter region of OCTN2 (that downregulates both basal transcription and transcription induced either by heat shock or arachidonic acid. This transcription difference is apparently due to the disruption of a binding site for heat shock transcription factor 1 (HSF1). A haplotype of two mutations is found in Crohn's Disease (CD) patients. Together, these mutations reduce cellular ability to respond to metabolic stress in inflamed tissue and may further be involved in the clearance of toxic substances from cells.