Neonatal severe hyperparathyroidism (NSHPT) causes severe hypercalcaemia, metabolic bone disease, and potential neurodevelop-mental deficits, all of which can be life-threatening. The use of calcimimetic agents can prevent or delay technically difficult parathyroidectomy in the newborn period. We present a 6-day-old male infant who presented with poor feeding, weight loss, and severe hypotonia. His total serum calcium and parathyroid hormone levels were very high (23.6 mg/dl and 1120 ng/dl, respectively). Based on these findings, the patient was diagnosed with NSHPT and was started on cinacalcet therapy until the genetic analysis results were available. Genetic analysis revealed a previously reported homozygous mutation in the CASR gene that was unresponsive to cinacalcet therapy in the literature. However, a normocalcaemic state unexpectantly occurred, which could be maintained with low calcium formula and cinacalcet therapy up to 13 months of age in the patient. Nevertheless, hypercalcaemia developed 2 months after he started a normal calcium-containing diet. Therefore, the patient underwent total parathyroidectomy at 17 months of age. We would like to emphasize, in light of this case, that cinacalcet treatment may be considered as first-line therapy for delaying parathyroidectomy in all cases with NSHPT, even in those who have an unresponsive cinacalcet CASR gene mutation.