Experimental evidence suggests that vitamin D might play a role in the development of breast cancer. Although the results of case-control studies indicate that circulating 25-hydroxyvitamin D [25(OH)D] is inversely associated with the risk of breast cancer, the results of prospective studies are inconsistent. A case-control study embedded in the European Prospective Investigation into Cancer and Nutrition (EPIC) was carried out comprising 1,391 incident breast cancer cases and 1,391 controls. Multivariable conditional logistic regression models did not reveal a significant overall association between season-standardized 25(OH)D levels and the risk of breast cancer (ORQ4-Q1 [95% CI]: 1.07 [0.85-1.36], p(trend) = 0.67). Moreover, 25(OH)D levels were not related to the risks of estrogen receptor positive tumors (ORQ4-Q1 [95% CI]: 0.97 [0.67-1.38], p(trend) = 0.90) and estrogen receptor negative tumors (ORQ4-Q1 [95% CI]: 0.97 [0.66-1.42], p(trend) = 0.98). In hormone replacement therapy (HRT) users, 25(OH)D was significantly inversely associated with incident breast cancer (ORlog2 [95% CI]: 0.62 [0.42-0.90], p = 0.01), whereas no significant association was found in HRT nonusers (ORlog2 [95% CI]: 1.14 [0.80-1.62], p = 0.48). Further, a nonsignificant inverse association was found in women with body mass indices (BMI)