Arising from: Waters AM et al. (2008) Tubulointerstitial nephritis as an extraintestinal manifestation of Crohn's disease. Nat Clin Pract Nephrol 4: 693–697 [doi:10.1038/ncpneph0955]

I read with interest the article by Waters and colleagues.1 The authors state that Crohn's disease is characterized by mucosal ulcerations; I disagree. The major types of inflammatory bowel disease are ulcerative colitis and Crohn's disease.2 Although the former is a nontransmural inflammatory disorder that is restricted to the colon, the latter is a transmural inflammatory disorder that can affect the entire gastrointestinal tract, and sometimes contribute to fistulas.