IgG4-related sclerosing disease is a rare disease characterized by fibrosis and lymphoplasmacytic infiltration in various organs. Here, we report a rare case of IgG4-related fibrosis that presented as a unilateral ureteral mass in a 39-year-old man who presented with abdominal pain. Left hydronephrosis and a mass measuring 3 × 1.1 cm in the lower portion of the left ureter were found. As a ureteral malignancy could not be ruled out, the left ureter was resected partially. Pathologically, severe fibrosis and infiltration of plasma cells, lymphocytes, and eosinophils were found. No malignancy was found. Immunohistochemically, most of the plasma cells were IgG4-positive. The serum IgG4 level was also elevated (233 mg/dl). The histological characteristics were similar to those of retroperitoneal fibrosis, inflammatory pseudotumor, or idiopathic segmental ureteritis. It is important to consider IgG4-related sclerosing disease in the differential diagnosis of a unilateral ureteral mass.

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