Low levels of 25-hydroxy vitamin D (25(OH)D) and polymorphisms in the vitamin D receptor gene (VDR) have been found separately to increase risk of breast cancer. The aim of this study was to determine whether low 25(OH)D levels, alone and in combination with BsmI VDR genotype, increased breast cancer risk in a United Kingdom (UK) Caucasian population. Breast cancer patients (n=179) and control women (n=179) were recruited and 25(OH)D levels measured by enzyme-linked immunosorbent assay (ELISA). VDR genotype was determined by polymerase chain reaction (PCR) and restriction enzyme digest. Analysis showed that subjects with 25(OH)D levels <50 nM and the bb BsmI VDR genotype are 6.82 times more likely to have breast cancer than subjects with levels of 25(OH)D>50 nM and either the BB or Bb genotype (95% confidence interval (CI) 2.31-14.7, P<0.001). This study indicates that low levels of circulating 25(OH)D, both alone and in combination with BsmI VDR genotype, may increase risk of breast cancer in a UK Caucasian population.