In a 32-year-old woman suffering from severe diarrhoea, eosinophilic infiltration of colonic mucosa and a peripheral eosinophilia, microbiological investigations only revealed large numbers of Enterobius vermicularis (pinworm) in the faeces. Treatment with mebendazole resulted in a rapid resolution of symptoms and disappearance of the eosinophilia, which strongly suggested a causative role of this pinworm in the clinical syndrome of the patient. E. vermicularis is generally regarded as an innocent nematode, which at most causes perianal pruritus due to migration of worms from the colon and expulsion of eggs onto the perianal skin. Although the pinworm maturates and lives in the gut, gastrointestinal symptoms have seldom been reported. E. vermicularis infection should be considered in patients with unexplained eosinophilic enteritis.