Hyperpigmented cutaneous lesions that flare-up with scratching: Darier's sign.

We present the case of a 40-year-old male with a past medical history of urticaria pigmentosa (UP) who presented for consultation to the Allergy clinic for an opinion of systemic mastocytosis. Previous workups included elevated serum tryptase level, UP on skin biopsy and an increased number of CD117-positive/CD25-positive mast cells on bone marrow biopsy. This case emphasizes the importance of physical findings such as Darier's sign in patients with hyperpigmented lesions, which virtually supports the diagnosis UP, raises the suspicion for systemic mastocytosis and guides further diagnostic evaluation.