OBJECTIVE: To evaluate relationships among vitamin D, proteinuria, and disease activity in pediatric systemic lupus erythematosus (SLE) and juvenile dermatomyositis (JDM). STUDY DESIGN: Multiple linear regression was used to associate subject-reported race, sunscreen use, and vitamin D intake with physician-assessed disease activity and serum 25-hydroxyvitamin D (25[OH]D) in 58 subjects with pediatric SLE (n=37) or JDM (n=21). Serum 25(OH)D was correlated with urinary vitamin D binding protein/creatinine ratio (DBP/C) and other indicators of proteinuria. RESULTS: Serum 25(OH)D levels in subjects with SLE were inversely associated with the natural log of urinary DBP/C (r=-0.63, P<.001) and urine protein to creatinine ratio (r=-0.60, P<.001), with an adjusted mean 10.9-ng/mL (95% CI, 5.1-16.8) decrease in 25(OH)D for those with proteinuria. Excluding subjects with proteinuria, serum 25(OH)D levels were inversely associated with disease activity in JDM, but not in SLE. Overall, 66% of all subjects were taking concurrent corticosteroids, but this was not associated with 25(OH)D levels. CONCLUSIONS: Low serum 25(OH)D in patients with SLE is associated with proteinuria and urinary DBP. Vitamin D deficiency is associated with disease activity in patients with JDM and SLE; this relationship in SLE may be confounded by proteinuria.