Calcifying fibrous pseudotumor is a recently described distinctive lesion, characterized by the presence of abundant hyalinized collagen with psammomatous or dystrophic calcifications and a lymphoplasmacytic infiltrate. The cause and pathogenesis are unclear, but a possible relationship with other pseudotumors, like nodular fasciitis or inflammatory myofibroblastic tumor, has been proposed by some authors. However, cases with overlapping histologic features have not been reported. A 17-year-old girl with multiple peritoneal calcifying fibrous pseudotumors and inflammatory myofibroblastic tumors (inflammatory pseudotumors) is described. Some multinodular lesions showed calcifying fibrous pseudotumors next to inflammatory myofibroblastic tumors. Transitional stages between calcifying fibrous pseudotumor and inflammatory myofibroblastic tumor were also present. This case clearly illustrates a histogenetic relationship between calcifying fibrous pseudotumor and inflammatory myofibroblastic tumor, and it suggests that calcifying fibrous pseudotumor is a late sclerosing stage of inflammatory myofibroblastic tumor, at least in some cases.