We plan to conduct a clinic-based interventional study of 30 patients followed at Brigham and Women's Hospital with history of nephrolithiasis, urinary calcium excretion between 200 and 400 mg/day, and 25-vitamin D deficiency (defined as serum level ≤ 25ng/ml). The intervention is oral ergocalciferol 50,000 IU per week for 8 weeks [1], and each participant will serve as his own control. The outcome is the change in urinary calcium excretion. The planned study duration is 3 months. We will have greater than 95% power to detect a 10% increase in 24-hour urinary calcium