IgG4-related disease (IgG4-RD) is a systemic immune-dependent pathology marked by infiltration of lymphocytes and plasma cells expressing IgG4 in affected tissues, leading to phlebitis and fibrosclerosis. In urological practice, diagnosing IgG4-RD of the ureter, which may resemble a malignant tumor, presents challenges. We present a unique case: a 64-year-old patient (Patient P.) experienced left lumbar discomfort and a 4 kg weight loss over 2 months. A computed tomography scan revealed a 111 mm tumor obstructing the left ureter, causing hydronephrosis and regional lymphadenopathy. Suspecting ureteral urothelial cancer, the patient underwent a left nephroureterectomy with lymphadenectomy. Microscopic analysis revealed fibrosis and inflammation infiltration (lymphocytes, plasma cells, and eosinophils) in the ureter wall, with no evidence of tumor growth. Immunohistochemistry confirmed IgG4-positive plasma cells. Serum IgG4 rose to 149 mg/dL. Morphological findings led to a diagnosis of IgG4-RD of the ureter. Clinically, it is crucial to recognize IgG4-RD in ureteral neoplasms for early detection, to prevent unnecessary surgical intervention.

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