Alterations within the oral cavity can be the first sign of systemic diseases and may thus allow for an early diagnosis and treatment. In particular, being the oral cavity a part of the gastrointestinal system, oral alterations can be an expression of a gastrointestinal disease. Diffuse mucosal swelling, cobblestone mucosa, localised mucogingivitis, deep linear ulceration, fibrous tissue tags, polyps, nodules, pyostomatitis vegetans, and aphthous-like ulcers have been described in Crohn's disease. A prompt recognition of systemic diseases through a careful examination of the oral cavity allows for proper investigations and timely treatment.