Although hypercalcaemia in young children or adolescents immobilized by fractures or spinal cord injury is well recognized, hypercalcaemia in adult immobilized patients is rare without a pre-existing bone disease. To our knowledge, hypercalcaemia in a head-injured, immobilized patient has not been previously reported. We report here a case where a previously normocalcaemic, immobilized, head-injured adult patient developed cognitive decline secondary to hypercalcaemia five months after injury, when transient interruption of enteral feedings led to mild dehydration. Indices of bone turnover were elevated and parathyroid hormone was appropriately suppressed. Possible predisposing factors in our patient included a severe degree of immobilization and a very high level of athletic activity prior to injury. Careful fluid management and specific monitoring of calcium levels, even several months post-injury, should be performed to avoid the added complications of hypercalcaemia in head-injured patients.