Vitamin D deficiency is prevalent in pregnant women. This deficiency could be exaggerated in multifetal pregnancies by the increased demand on maternal stores of vitamin D. We present 2 cases in which hypocalcemia and secondary hyperparathyroidism occurred in 2 sets of twins from mothers with vitamin D deficiency. The first mother had gastric bypass surgery and Crohn disease. The second mother had no apparent cause of vitamin D deficiency. Both women had iron deficiency anemia and lived in Northeastern United States. We speculate that in twins, the demand for 25-hydroxyvitamin D by 2 fetuses could deplete the 25-hydroxyvitamin D stores in a mother.