This prospective study will assess if 12 months of vitamin D3 (cholecalciferol) supplementation, in patients with AAV (GPA, MPA, and EGPA) who have deficient or insufficient 25(OH)D3 status at enrollment, correlates with improved disease activity and/or lower frequency of relapse (compared to historical data and a previously conducted cross sectional study (part I) that assessed vitamin D status in a cohort of similar patients).

1. Subject cohorts: Patients with a diagnosis of AAV from the MSH Rheumatology clinic in a cohort of consecutive patients over a 3 month recruitment period.
2. Study Design, Data Collection: Patients with insufficient and/or deficient vitamin D status at study enrolment will be instructed to take 12 months of vitamin D3 (cholecalciferol) supplementation (1000 IU for those not on vitamin D, or to increase the dose up to 2000 IU per day for those already on vitamin D) vs. those with sufficient status who will be observed for 12-months.
3. Study Duration: 12 months.
4. Study Endpoints: Vitamin D status vs. disease activity.

Inclusion criteria:

• Patients at least 18 years of age, women and men
• Followed at the Mount Sinai Hospital, Vasculitis clinic, Toronto
• With a diagnosis of AAV (GPA, MPA or EGPA) satisfying the modified American College of Rheumatology 1990 classification criteria and/or the revised 2012 Chapel Hill nomenclature definition.

Exclusion criteria:

• Current or history of hypercalcemia, primary hyperparathyroidism, sarcoidosis, hypervitaminosis D, Williams syndrome, other autoimmune, chronic inflammatory or infectious conditions, malabsorptive disorders, cancer, type 1 diabetes, liver disease
• Current or planned pregnancy within the next year.
• Intolerance, allergy to oral vitamin D, and/or contra-indication to take oral vitamin D