Vitamin D deficiency may increase the risk for metabolic syndrome. We determined the relationship of serum 25-hydroxyvitamin D (25(OH)D) with metabolic syndrome components in obese adolescent females and assessed whether vitamin D treatment corrects metabolic disturbances. Eighty postmenarchal adolescents (53 African American (AA) and 27 Caucasian American (CA)) were evaluated with blood pressures and fasting measurements of serum 25(OH)D, lipid profile, C-reactive protein, alanine transaminases (ALTs) and aspartate transaminases followed by an oral glucose tolerance test. A subgroup (n = 14) of vitamin D deficient subjects were re-evaluated following vitamin D treatment. Among all subjects, 25(OH)D was inversely associated with fasting glucose (r =-0.28, P = 0.02) and positively associated with low-density lipoprotein (LDL) cholesterol (r = 0.31, P = 0.008), independent of race and BMI. In analyses by race, adjusted for BMI, 25(OH)D was inversely associated with fasting insulin in CA (r =-0.42, P = 0.03) but not AA (r =-0.11, P = 0.43) whereas 25(OH)D was positively associated with ALT in AA, but not CA (r =-0.29, P = 0.04 vs. r = 0.21, P = 0.32). Fasting glucose improved in vitamin D treated subgroup (from 89.07 8.3mg/dl to 84.34 8.4mg/dl, P = 0.05). A trend toward improvement in fasting glucose remained after exclusion of four subjects whose serum 25(OH)D 2 did not improve following treatment (P = 0.12). In conclusion, serum 25(OH)D was inversely associated with fasting glucose, and vitamin D treatment had beneficial effects on fasting glucose. Relationships of 25(OH)D with fasting insulin and ALT were ethnic specific. The positive relationship with LDL and ALT were suggestive of possible adverse influences of vitamin D. © 2011 The Obesity Society.