A 58 year old woman presented with unilateral visual loss due to ischaemic optic neuropathy. Investigations revealed severe hypercalcaemia due to hyperparathyroidism. Excision of a large parathyroid adenoma restored normocalcaemia. Visual acuity was unchanged. There was no evidence of any condition known to be associated with ischaemic optic neuropathy, or of any abnormal clotting tendency. The association between primary hyperparathyroidism and ischaemic optic neuropathy may be causal and related to vascular damage mediated by hypercalcaemia.