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A 26-year-old woman with Down syndrome presented with a 4-cm-sized palpable mass in the sacrococcygeal region. Histologic evaluation of the specimen revealed densely packed collagen fibers in the dermis, which is consistent with a collagenoma. Masson trichrome staining showed dense fibrosis, and elastic Van Gieson staining revealed a significant decrease in elastic tissue. The clinicopathologic characteristics of this patient and 4 previously reported cases are discussed in detail.