In May, 2008, a 41-year-old man was referred to our allergy clinic with recurrent spells of severe fl ushing, malaise, nausea, palpitations and dizziness, with no known trigger. Since the fi rst attack in April, 1988, he had consulted several doctors, undergoing extensive medical assessment, which excluded metabolic, cardiovascular, psychological, and neurological disease. From the outset the patient had recorded 97 attacks and classifi ed them according to severity (fi gure). Most incidents were mild, but he was taken to the emergency room needing epinephrine or corticosteroids six times, was occasionally unconscious, with faecal incontinence, and was admitted to intensive care twice. The patient described an episode as “Everything begins with a vague feeling of nausea, followed rapidly by an increase in heart rate. My pulse can become as high as 150 beats per min”. Initial symptoms often lasted 10 min, and were followed by tingling of the tongue, sweating, and fl ushing of the face and the trunk. Occasionally he had severe vomiting, and on some occasions a low body temperature (35·7°C) was recorded on examination in hospital. The duration of symptoms varied, with a severe attack lasting more than 30 min. Afterwards he often had severe headache, chills, persistent nausea, and swelling of the mouth and tongue, lasting for several hours. Seizures were never reported. Our initial investigations revealed no allergy, but he had raised baseline serum tryptase of 160 ng/mL (reference ≤11·4 ng/mL), increased urinary concentrations of 11β-PGF2α (127 ng/mmol creatinine; reference ≤35 ng/mmol creatinine) and leukotriene E4 (131 ng/mmol creatine; reference ≤70 ng/mmol creatine), supporting an underlying mast-cell-activation disease. Bone marrow biopsy showed numerous mast cell aggregates, spindleshaped mast cells, an aberrant mast cell phenotype expressing CD2/CD25, and KIT D816V mutation. Based on these fi ndings, we diagnosed systemic mastocytosis. Since June, 2009, the patient has been taking prophylactic desloratadine (15 mg daily), ranitidine (300 mg daily), and montelukast (20 mg daily). At follow-up in February, 2014, he reported 56 months without a severe reaction (fi gure), and 39 months without any attacks at all. He now carries an epinephrine syringe at all times. Mastocytosis is characterised by the accumulation, activation, and proliferation of mast cells in various organs. Symptoms are typically episodic and associated with the release of mast cell mediators. One of the most common presentations is anaphylactic symptoms that recur in a self-limiting and stereotypical fashion. Consequently, many patients with mastocytosis are initially admitted for anaphylaxis. The diff erential diagnosis of spells is challenging, and this case exemplifi es a common problem in patients with mastocytosis. Despite the rarity of this condition, general practitioners and emergency physicians should be aware of systemic mastocytosis as a possible cause of recurrent, indeterminate spells and anaphylaxis. Although spells in mastocytosis are due to mast cell mediators, current prophylactic treatment usually consists of antihistamine alone, leaving other potent mediators unopposed. It is worth noting that in this case combined histamine and leukotriene-receptor antagonism has so far prevented the recurrence of severe reactions.