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A process of evolution and involution is a characteristic feature of reactive fibrous lesions, but has not been emphasized in either the radiology or pathology literature. We report a case of proliferative fasciitis, pseudotumorous fibrous proliferation similar to nodular fasciitis, showing evolution and involution during a 1-month period. A 35-year-old Japanese woman presented with a tender soft tissue mass in the forearm that had grown over a 2-week period. MR imaging revealed a soft tissue mass of nonspecific signal intensity on the muscle fascia. Needle biopsy revealed diffuse proliferation of fibroblastic spindle cells representing proliferative fasciitis. Two weeks after biopsy the mass had rapidly decreased in size, and the pain subsided. MR imaging obtained 1 month later demonstrated only minimal residual reactive change along the fascia. Conservative treatment may be a reasonable treatment option for this condition.