Background: Circulating 25-hydroxyvitamin D [25(OH)D] and parathyroid hormone (PTH) concentrations have been shown to be associated with insulin sensitivity; however, adiposity may confound this relation. Furthermore, African Americans (AAs) have lower insulin sensitivity and 25(OH)D concentrations than do European Americans (EAs); whether these differences are associated in a cause-and-effect manner has not been determined. Objectives: The objectives of this study were to examine the relation of 25(OH)D and PTH concentrations with whole-body insulin sensitivity and to determine whether lower 25(OH)D concentrations in AAs compared with EAs contribute to the lower insulin sensitivity of AAs relative to that of EAs. Design: This was a cross-sectional study of 25 AA and 25 EA women. We determined the whole-body insulin sensitivity index (SI) with an intravenous glucose tolerance test and minimal modeling. Percentage body fat was determined with dual-energy X-ray absorptiometry, and intraabdominal adipose tissue (IAAT) was determined with computed tomography. Results: Multiple linear regression analysis indicated that 25(OH)D and PTH concentrations were independent determinants of SI [standardized β = 0.24 (P = 0.04) and 20.36 (P = 0.002), respectively] after adjustment for age, race, and IAAT. The mean ethnic difference in SI decreased from 2.70 [· 10-4 · min-1/(μIU/mL)] after adjustment for IAAT and percentage body fat to 1.80 [· 10-4 · min-1/(μIU/mL)] after further adjustment for 25(OH)D and PTH concentrations. Conclusions: 25(OH)D and PTH concentrations were independently associated with whole-body insulin sensitivity in a cohort of healthy women, which suggested that these variables may influence insulin sensitivity through independent mechanisms. Furthermore, ethnic differences in 25(OH)D concentrations may contribute to ethnic differences in insulin sensitivity. © 2010 American Society for Nutrition.