Systemic mastocytosis (SM) is an acquired myeloproliferative disease, which is caused by an uncontrolled proliferation of aberrant mast cells. SM patients can have very different clinical phenotypes and may therefore initially present to different specialties. Diagnosis is often delayed because many physicians are unfamiliar with this illness. This can lead to substantial morbidity and puts patients at risk of complications such as severe anaphylaxis. Measurement of serum tryptase levels is always a sensible first step in the diagnostic work-up, but a normal serum tryptase does not rule out SM completely, and a bone marrow biopsy is essential for a conclusive diagnosis. Here, we describe two patient cases to illustrate the heterogeneous nature of this disease, and provide an overview of the symptoms, diagnostic work-up and current treatments options for SM.