OBJECTIVES: To describe the level of serum vitamin D (25(OH)D) inadequacy by gender and race among US adults using national level data METHODS: Cross-sectional study was conducted using the National Health and Nutrition Examination Survey (NHANES) from 2001 to 2006, i.e., the latest data including serum 25(OH)D concentration, including all US adults (>=40 years).  Predictive factors of serum 25(OH)D inadequacy (i.e., <20 ng/ml) were evaluated using study variables including patient demographic, health and lifestyle factors, health care utilization,  insurance coverage, and clinical comorbidities.  All analyses were performed with SAS statistical software, version 9.1, at an alpha of 0.05 RESULTS: Of 125 million adults, 37.3% had inadequate serum 25(OH)D levels. The inadequacy was higher in female than male participants (40.6% vs 33.6%: p<0.0001, respectively). Among race/ ethnicity groups, the prevalence of serum 25(OH)D inadequacy was significantly higher in non-hispanic black populations (77.6%), hispanic populations (50.5%), and other race (53.2%) than non-hispanic whites (29.7%) (p<.0001).  Participants who had no health insurance coverage was associated with the higher prevalence of serum 25(OH)D inadequacy (47.4% vs. 35.9%: p<0.0001). CONCLUSIONS: A significant number of US adults maintain inadequate serum 25(OH)D level.  The prevalence of the inadequacy was significantly higher in female participants and black populations as compared to their counterparts. Coordinated efforts through comprehensive programmatic approaches or improved collaboration including other health care professionals such as pharmacists and nutritionists can be a key element to improve vitamin D adequacy in US health care system.