Mastocytosis is a relatively infrequent disorder characterized by mast cell proliferation within primarily the skin, but also various organs such as the bone marrow, liver, spleen, lymph nodes and the gastrointestinal system. Several factors are known to induce symptoms in patients with mastocytosis. A 13-year-old boy with cutaneous mastocytosis and type 1 diabetes mellitus developed multiple itchy papules 2 to 3 days after he started receiving insulin (lispro) pump therapy. Punch biopsy revealed widespread mast cell infiltration on the papillary dermis. We hypothesized that the insulin pump catheter caused a physical stimulus due to local microtrauma and resulted in the formation of the lesions. We, therefore, discontinued insulin pump treatment and switched to SC insulin therapy. While previous lesions healed successfully, no new lesions occurred. To confirm our hypothesis, we repeated the procedure and observed the occurrence of similar lesions around the insulin pump catheter. Herein, we present for the first time, a patient with type 1 diabetes mellitus and cutaneous mastocytosis who developed exacerbation of lesions during insulin pump therapy.