Vitamin D insufficiency is prevalent and may be associated with higher risk for metabolic syndrome. Low serum 25-hydroxyvitamin D3 is known to perturb cellular function in many tissues, including the endocrine pancreas, which are involved in the pathogenesis of obesity and type 2 diabetes. This study examined the vitamin 25(OH)D3 concentration and its relationship with the metabolic syndrome among 52 young women aged 20-40 yrs with overweight and obese. As defined by revised International Diabetes Federation (IDF 2005) criteria, 27 of the 52 women had the metabolic syndrome (52%). Women with MS had significantly lower mean concentration of vitamin 25(OH)D3. Vitamin D insufficiency was more prevalent in women with MS, compared with those who did not fulfill the criteria for this syndrome (63% vs 37%, respectively) as well as among women with metabolic syndrome mild deficiency occurred much more frequently than in without MS (58% vs 26%, respectively). When serum concentrations of 25(OH)D3 were categorized in tertiles, there was a decreasing prevalence of MS in women with increasing concentrations of 25(OH)D3. The study findings suggest that insufficiency of vitamin 25(OH)D3 is more common in women with excessive body weight and metabolic syndrome than in women with excessive body weight without metabolic syndrome.