Eosinophilic esophagitis, gastroenteritis and colitis are very likely underdiagnosed conditions, and their actual incidence may be increasing. The diagnosis and treatment remain, however, fairly poorly established and are based on inadequate scientific evidence. Differential diagnosis is broad, mainly conditions causing secondary eosinophilia, such as allergies. If the secondary causes have been carefully excluded, the possibility of rare hypereosinophilic syndrome should be considered in prolonged eosinophilia.