To document the point that the hyperparathyroidism should be considered a possible cause of unexplained neurological and psychiatric symptoms, the authors present five case reports of confirmed primary hyperparathyroidism in which the patients initially appeared with problems that seemed mainly psychiatric. The presenting symptoms in these cases consisted of varying degrees of depression, catatonia, confusion, disorientation, fatigue, and lethargy; there was no associated bone or renal pathology in four of the cases. The authors include a review of the pertinent literature and a discussion of the effect of calcium and magnesium bivalent ions on the central nervous system associated with hyperparathyroidism. They conclude that more investigation of the role of magnesium in this disease seems warranted.