A 76-year-old man with a 52-year history of urticaria pigmentosa was found to have an elevated serum tryptase level as well as osteoporosis. Histopathologic alterations of one his skin lesions showed an infiltrate of mast cells. Urticaria pigmentosa patients are at risk for osteoporosis because of elevated heparin and stem-cell factor levels. These patients should be screened with serum tryptase levels and bone density studies to detect osteoporosis, regardless of their age.