IgG-4 related disease (IgG4-RD) is a chronic inflammatory condition characterized by tissues infiltration with lymphocytes and IgG4 secreting plasma cells, significant fibrosis and response well to steroids. It is typically a systemic disease that involves various organs. We are presenting a 57-year-old female who was found to have elevated serum creatinine, Proteinuria. An ultrasound scan revealed bilateral hydronephrosis and an abdominal computed tomography (CT) scan illustrated a large retroperitoneal mass. Further evaluation of the mass with whole-body positron emission tomography (PET CT) scan was suggestive of retroperitoneal fibrosis or lymphoma. A CT guided biopsy was done, and results were consistent with fibrosis in the context of IgG-4 related disease. IgG-4 serum levels were elevated. The patient was treated with oral steroids. Azathioprine and Rituximab. Renal function improved significantly and was stable upon 3 months follow up with a decrease in IgG-4 levels and with a significant reduction in the mass lesion.