Primary hypoparathyroidism caused by lymphocytic parathyroiditis was diagnosed in a cat. Other causes of hypocalcemia (ethylene glycol toxicosis, phosphate enema administration, pancreatitis, renal insufficiency, and malabsorption) were ruled out on the basis of history, clinicopathologic data, and lack of supportive clinical signs, which in this cat included inappetence and tetanic muscle spasms. The diagnosis was confirmed by histologic examination of a surgically excised thyroparathyroid lobe that comprised lack of recognizable parathyroid tissue and a lymphocytic plasmacytic infiltrate adjacent to the cranial pole. A treatment regimen similar to that for iatrogenic postthyroidectomy hypoparathyroidism was successful in controlling clinical signs of the disease.