To study hyperplasia of peptidergic nerves purported to be diagnostic of Crohn's disease, we determined the distribution and concentrations of gut neuropeptides in specimens of normal intestine, ulcerative colitis, and Crohn's disease. Tissue specimens obtained at surgery were dissected into the mucosal-submucosal and muscularis externa layers, and immunoreactive gut neuropeptides were acid-extracted for measurement by radioimmunoassay. The immunoreactive species were characterized by column chromatography. Mucosal-submucosal layer concentrations of vasoactive intestinal peptide were significantly decreased in Crohn's colitis and ulcerative colitis, while mucosal-submucosal layer concentrations of substance P were significantly increased in left-sided ulcerative colitis. Muscularis externa layer concentrations of vasoactive intestinal peptide and met5-enkephalin were decreased in left-sided Crohn's colitis. These neuropeptide concentration abnormalities did not clearly differentiate between Crohn's colitis and ulcerative colitis, and no increase in concentration of a neuropeptide diagnostic of Crohn's disease was identified.