A 4 year old castrated male Cairn Terrier presented to Cornell University Hospital for Animals’ Emergency Service in December 2014 as a referral for painful, swollen joints and increased liver enzymes. He originally presented to his primary care veterinarian as he had become acutely lethargic and his gait was described as “walking on egg shells”. His physical exam revealed pain in multiple joints. A 4Dx Snap Test was negative and he was given a week course of carprofen. The dog did not improve on carprofen and was started on a trial of doxycycline. Over the next two weeks, the patient continued to feel lethargic, stiff and he became inappetant. On presentation, the patient had painful, effusive joints, with the carpal joints being most severely affected. He was febrile and had a short-strided gait in all four limbs. He had mildly enlarged popliteal and superficial cervical lymph nodes bilaterally. The rest of his physical exam was unremarkable. Further diagnostics included bloodwork, an antinuclear antibody test, arthrocentesis with cytology and culture, radiographs, and an abdominal ultrasound. Diagnostics ruled out underlying primary diseases, however, the cytological results of the joint fluid were inconsistent with the typical immune-mediated polyarthritis findings. These collective findings were consistent with idiopathic immune-mediated polyarthritis. The dog responded favorably to prednisone, mycophenolate and pain medications. This case report reviews etiologies of immune-mediated polyarthritis, the necessary diagnostics and different treatment options.