Severe hypocalcaemia in breast cancer with bone metastasis is a rare finding usually associated with an advanced stage of the disease. We report a case of a 45-year-old woman with a history of local ductal carcinoma in situ (DCIS) of the breast, who presented with muscle tremors and general weakness. Hypocalcaemia was evident, with a positive Chvostek sign and a serum calcium level of 5.9 mg/dL (1.47 mmol/L), phosphorus 5.9 mg/dL (normal range: 2.3–4.7 mg/dL) with normal levels of albumin, magnesium and parathyroid hormone. High oral doses of alpha calcitriol and calcium with i.v. infusion of high calcium doses were instituted, altogether sufficient to maintain only mild hypocalcaemia. A whole-body CT revealed bone lesions along the axial skeleton. A biopsy from a bone lesion revealed a metastasis of breast carcinoma. With this pathological finding, leuprolide (GNRH analogue) and chlorambucil (alkylating agent) were initiated, followed by prompt tapering of infused calcium down to full discontinuation. Serum calcium was kept stable close to the low normal range by high doses of oral alpha calcitriol and calcium. This course raises suspicion that breast metastases to the skeleton caused tumour-induced hypocalcaemia by a unique mechanism. We assume that hypocalcaemia in this case was promoted by a combination of hypoparathyroidism and bone metastasis. Learning points Severe hypocalcaemia can a presenting symptom for breast cancer relapse.