Enteroviruses commonly infect the gastrointestinal tract, and replication of enteroviruses has been well documented in the Peyer patches of the small bowel. Chronic enterovirus infection has been found in the stomach and terminal ileum of patients with myalgic encephalomyelitis/chronic fatigue syndrome. The authors report the unexpected finding of enterovirus VP1 protein, by immunoperoxidase staining, in carcinoid tumours found in one patient with myalgic encephalomyelitis/chronic fatigue syndrome and another patient with chronic lower quadrant abdominal pain, and suggest a possible association between enteroviruses and tumorigenesis.