Alterations of the oral cavity are common in children: 22% of children aged less than 4 years and 44% of those aged more than 12 develop dental erosion, 9-95% of children in Europe and in North America develop gingivitis, with adolescents showing a prevalence of more than 60% [Italian Ministry of Health, Guidelines 2013]. 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. Pyostomatitis vegetans can be a sign of ulcerative colitis. Dental erosion with enamel loss in facial, occlusal, and lingual surfaces, and an increased risk of dental caries have been reported in children and adolescents with gastro-oesophageal reflux with varying prevalence. A prompt recognition of systemic diseases through a careful examination of the oral cavity could allow proper investigations and management in a timely fashion.