Temporally associated with the improvement in vitamin D nutrition in many Western countries in the mid-20th century, there was a change in many characteristics of primary hyperparathyroidism. Osteitis fibrosa cystica became a rare manifestation of what is now frequently an asymptomatic disease. At the same time, in patients with the disease, levels of PTH and parathyroid adenoma weights have fallen dramatically. In view of these observations and others, an association between vitamin D deficiency and severity of primary hyperparathyroidism has been proposed. Data support an association on two distinct levels. First, regardless of the clinical severity of primary hyperparathyroidism, the disease seems to be more severe in those with concomitant vitamin D deficiency. Second, vitamin D deficiency and insufficiency seem to be more prevalent in patients with primary hyperparathyroidism than in geographically matched populations. The association between vitamin D deficiency and primary hyperparathyroidism has clear implications. Co-existing vitamin D deficiency may cause the serum calcium level to fall into the normal range, which can lead to diagnostic uncertainty. With regard to management, preliminary data on vitamin D repletion in patients with mild primary hyperparathyroidism suggest that, in some cases, correction of vitamin D deficiency may be accomplished without worsening the underlying hypercalcemia. Vitamin D–deficient patients undergoing parathyroidectomy are also at increased risk of postoperative hypocalcemia and “hungry bone syndrome,” which underscores the importance of preoperative assessment of vitamin D status in all patients with primary hyperparathyroidism.