A 53 year old woman presented with abnormal liver function tests and subsequently developed intermittent abdominal pain, vomiting and diarrhoea. There were no rash or anaphylactoid reactions. Endoscopic biopsies showed excessive density of eosinophils and immunohistochemical staining for tryptase revealed a florid mast cell infiltrate. A diagnosis of systemic mastocytosis was made by bone marrow biopsy. Systemic mastocytosis is a rare myeloid neoplasm often associated with gastrointestinal symptoms due usually to mediator release but may rarely represent organ infiltration. While endoscopic and routine biopsy appearances are non-specific, suggestive features should lead to staining for mast cell tryptase or CD 117. However, diagnose generally requires bone marrow biopsy. The prognosis in the majority of patients is good and supportive management only is required. For patients with aggressive disease, cytoreductive therapy may be needed.