A 6-year-old castrated male Labrador retriever was presented to the Cornell University Hospital for Animals Emergency Service for lethargy, shifting leg lameness, and possible kidney disease. The patient had a two-week history of weight loss, a one-week history of right forelimb lameness, and a three-day history of unwillingness to stand or walk. On presentation, the patient was depressed with generalized subcutaneous edema, multiple joint effusion, generalized lymphadenopathy, and left hindlimb lameness. Bloodwork revealed non-regenerative anemia, an inflammatory leukogram, azotemia, hyperphosphatemia, hypoalbuminemia, hypocalcemia, and hypercholesterolemia. Urinalysis revealed proteinuria and a protein-creatinine ratio of 13.7. Differential diagnoses included neoplasia, infection, and inflanunatory disease among others. Further diagnostics were unable to find an infectious or neoplastic etiology and the patient was diagnosed with an idiopathic immune-mediated polyarth:ropathy and a secondary protein-losing glomerular nephropathy. This report discusses canine immune-mediated polyarthritis and protein-losing glomerular nephritis including an overview ofpathophysiology, diagnosis, treatment, and prognosis.