Primary hyperparathyroidism (PHPT) can cause hypercalcaemia secondary to a pathologically high secretion of parathyroid hormone. Rarely this can first manifest as acute psychosis. It is imperative to exclude organic causes of psychosis before labelling the psychosis as primarily psychological. If hypercalcaemia is revealed, investigation is required to elucidate the underlying cause whilst instigating treatment to lower serum calcium levels. If PHPT is the underlying pathology, subsequent treatment involves surgical exploration and resection of the parathyroid adenoma or hyperplasia.