An increasing number of patients are diagnosed with primary hyperparathyroidism after having hypercalcaemia detected incidentally during routine biochemical screening. Many are asymptomatic at the time of diagnosis. An 80-year-old woman was found to have asymptomatic hypercalcaemia. Initial investigations suggested a diagnosis of primary hyperparathyroidism. Subsequent investigations revealed that, in fact, she had familial hypocalciuric hypercalcaemia. Direct DNA sequencing of the calcium-sensing receptor (CASR) gene confirmed that the patient was heterozygous for c.2501delC, a novel frame shift mutation predicted to cause loss of function of the CASR gene. Several other family members were subsequently found to carry the same mutation. Suspected cases of hypocalciuric hypercalcaemia should be confirmed by detection of mutations within the CASR gene. Establishing the correct diagnosis will enable the patient and family members to avoid unnecessary investigations or operations.