Background: Cutaneous involvement occurs in most patients with systemic mastocytosis. Objective: We sought to determine whether the extent of cutaneous involvement is predictive of systemic disease. Methods: In a prospective survey of 48 adults and 19 children, the extent and density of cutaneous lesions were compared with patient history, symptoms, internal organ involvement, serum total mast cell tryptase level, and bone marrow pathology. Results: Cutaneous lesions in children were of a greater mean and maximum diameter, but similar in extent and density compared with lesions in adults. In adults with skin lesions, the extent of lesions correlated to disease duration. Adults with extensive cutaneous disease experienced more pruritus and flushing. Fatigue, splenomegaly, and hepatomegaly were more frequent in adults without cutaneous involvement; and in those with a greater density of lesions and disease duration. Increased tryptase levels were found in children and adults with systemic disease and correlated to skin lesion density and bone marrow pathology. Conclusion: An examination of the extent and density of cutaneous lesions in adults helps identify those with more extensive extracutaneous disease and, thus, requiring a more thorough evaluation. (J Am Acad Dermatol 2003;48:508-16.)