The investigators hypothesize that a 3 month course of vitamin D supplementation to treat 25(OH)D deficiency in stone formers with high levels of 24-hour urinary calcium will not increase urinary calcium excretion by greater than 10%.

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

Inclusion Criteria:

• History of nephrolithiasis
• 25(OH)D deficiency (defined as serum level ≤ 25ng/ml) within 3 months of enrollment
• 24-hour urinary calcium excretion > 199 mg/day and < 400 mg/day (measured less than 6 months prior to study enrollment)

Exclusion Criteria:

• Non-Caucasian
• Women of child-bearing age (age < 50)
• Known uric acid, cystine, or struvite stone disease
• Hypercalcemia (serum calcium > 10.4 mg/dl within the past 12 months)
• Gross hematuria within the past 6 months
• Acute stone event within the past 2 months
• Recent stone intervention within the past 1 month
• Suspected or known secondary causes of hypercalciuria such as primary hyperparathyroidism, sarcoidosis, hyperthyroidism, or malignancy (except non-melanoma skin cancer)
• Addition or dose change of medicines potentially affecting urinary calcium since enrollment 24-hour urine collection (including diuretics, magnesium supplements, potassium supplements, potassium citrate, and vitamin D supplementation)