We describe an elderly patient with primary hyperparathyroidism and dementia who was admitted to a psychogeriatric department because of delusions of new onset. While hospitalized he had three episodes of delusions that coincided with mild hypercalcemia. Treatment for the hypercalcemia resulted in rapid remission of the delusional state, but did not induce any change in his cognitive abilities or in the severity of dementia. We review the relevant literature and offer recommendations for treatment and follow-up of elderly patients with hyperparathyroidism associated with a delusional state.