A nine and a half year old spayed female Boxer presented to Cornell University Small Animal Emergency Service for an episode of sudden collapse during which she appeared to have flaccid muscle tone and appeared not to be breathing to her owners. She was diagnosed with a cardiac arrhythmia by her primary care veterinarian and referred to Cornell; at the Cornell Emergency Service, an EKG showed ventricular tachycardia with pulse deficits and initial bloodwork showed a hypercalcemia of ionized calcium and elevated cardiac troponin levels. Given her signalment and breed, the patient was hospitalized and worked up by the Oncology Service for a neoplastic origin of the hypercalcemia - diagnostics included thoracic radio graphs, abdominal ultrasound, splenic and bone marrow aspiration and a malignancy panel which showed no evidence of neoplasia. Primary hyperparathyroidism was suspected and confirmed with a cervical ultrasound which showed a nodule in the cranial pole of the right parathyroid gland. Surgical parathyroidectomy was recommended as treatment and the patient was booked in for a surgery a few weeks after initial presentation. While hospitalized, the patient also displayed odd neurological symptoms of titubation and lowering her head with eyes closed so a thorough neurological examination was performed which neurolocalized a presumptive left forebrain lesion. The presenting complaint of syncope was worked up with an echocardiogram which showed a mildly enlarged left atrium, appropriate systolic function and no congenital defects. Given the patient's breed and signalment, arrhythmogenic right ventricular cardiomyopathy (ARVC) was suspected as the underlying cause so the dog was started on anti-arrhythmic treatment with sotalol. She was also given a Holter monitor which recorded no events during the period of use. The patient experienced more syncopal episodes before returning to the Cornell Hospital; her worst was the morning before re-presentation when she took an hour to recover and lost bladder control. Upon re-admission to the hospital, she went into cardiopulmonary arrest. She was resuscitated but euthanized due to poor prognosis/quality of life concerns and submitted for necropsy. Necropsy confirmed findings of ARVC and a parathyroid nodule but no brain lesions. This paper will describe clinical findings, diagnostics and treatment in a dog with hypercalcemia and Boxer cardiomyopathy.