A 50-year-old man with no medical history presented with an asymptomatic rash on his chest, arms, abdomen, and back for 5 years. The patient reported increasing redness under hot water without associated pruritus. He denied any systemic symptoms. Examination showed monomorphic erythematous macules and papules distributed diffusely over the chest, upper extremities, abdomen, and back. Biopsy revealed mast cells concentrated in the dermal papillae which was confirmed with Giemsa and CD117 staining. Bloodwork was significant for an elevated serum tryptase to 27.6 ng/mL. The combination of clinical, histopathologic, and laboratory findings confirmed the diagnosis of diffuse cutaneous mastocytosis. The patient was recommended to avoid triggers for mast cell degranulation such as heat, sunlight, stress, medications, and alcohol. Additionally, he was referred to hematology for bone marrow biopsy, which was deferred due to lack of systemic symptoms. Mastocytosis is a spectrum of diseases with various cutaneous and systemic manifestations. Cutaneous findings include flushing, pruritus, urticaria, and bullae. Systemic symptoms include bone pain, headaches, fatigue, nausea, vomitus, diarrhea, weight loss, cramping, chest pain, palpitations, and syncope. Diffuse cutaneous mastocytosis is a rare entity usually presenting in childhood low risk of systemic involvement. However, when presentation occurs in adulthood, systemic involvement should be evaluated with serum tryptase and bone marrow biopsy when indicated. Without a bone marrow biopsy, the risk of systemic involvement can be estimated from symptoms and a tryptase level. Based on clinical presentation and laboratory findings, physicians must consider working up systemic involvement to avoid morbidity and mortality.