Less is known about gastrointestinal (GI) involvement of primary skin diseases due to the difference in embryology, histology, microbiology and physiology between integument and alimentary tract. Esophagus, following the oropharyngeal mucosa, is the most common GI segment affected by primary skin diseases, especially by eosinophilic esophagitis, lichen planus, and autoimmune bullous dermatoses like pemphigus vulgaris, mucosal membrane pemphigoid and epidermolysis bullosa acquisita. Eosinophilic esophagitis is an emerging chronic atopic disease with esophageal dysfunction as the typical presentation, and esophageal narrowing, rings and stricture as late complications. Esophageal lichen planus mainly involves the proximal to mid-esophagus in elderly-aged women with long-term oral mucosal lesions. In acute attack of pemphigus vulgaris esophageal involvement is not uncommon but often neglected and may cause sloughing esophagitis (esophagitis dissecans superficialis) with acute GI bleeding in rare cases. GI manifestation of hereditary bradykininergic angioedema with colicky acute abdomen mostly affects small intestine, usually in the absence of pruritus or urticaria, and is more severe and long-lasting than the acquired histaminergic form. Strong evidence supports association between inflammatory bowel disease, especially Crohn disease, and hidradenitis suppurativa/acne inversa. Patients with vitiligo need surveillance of autoimmune liver disease, autoimmune atrophic gastritis or celiac disease when corresponding symptoms become suspect. Melanoma is the most common primary tumor metastatic to the GI tract, with small intestine predominantly targeted. Gastrointestinal involvement is not uncommon in disseminated mycosis fungoides. Extramammary Paget’s disease is an intraepidermal adenocarcinoma of controversial origin and a high association between the ano-genital occurrence and colorectal adenocarcinoma has been reported. As GI tract is the largest organ system with multidimensional functions, dermatologists in daily practice should be aware of the gastrointestinal morbidities related to primary skin diseases for an early diagnosis and treatment.This article is protected by copyright. All rights reserved.