A 30-year-old Caucasian man presented with a recurring rash over the past several months. Pruritus was noted during exercise or overheating. Multiple reddish-tan macules and slightly raised papules, 7-12 mm in diameter, were visible on the patient’s trunk and extremities; his palms, soles, face, and scalp were spared. Rubbing or scratching caused the lesions to become erythematous, raised, and itchy. Laboratory studies were within normal limits, including a serum tryptase level.