The most recent hypothesis of the development of small vessel vascular dementia (VaD) emphasises the role of blood‐brain barrier (BBB) dysfunction. It is hypothesised that certain genetic polymorphisms of the BBB tight junction claudin‐1 protein, in combination with adverse environmental risk factors, increase the risk of BBB dysfunction and small vessel VaD. In this case‐control study, 97 control participants, with a mean Mini Mental State Exam (MMSE) score of 29.1, and 38 VaD participants were recruited and completed a questionnaire on their medical history and lifestyle factors. Blood was also collected and two single nucleotide polymorphisms (SNPs), rs17501010 and rs893051 of claudin‐1 genotyping, were analysed by real‐time polymerase chain reaction (PCR) assay. A significantly higher frequency of all rs893051 SNP genotypes (GC and CC) was found in the VaD population (OR=4.8, P=0.006 and OR=6, P<0.001 respectively). Patients with TT genotype of rs17501010 were also more likely to have VaD (OR=3.25, P=0.022). Stratification analysis revealed that having combined haplotype GC+CC of rs893051 and lipid disorders was associated with higher risk of VaD (OR=9.9, P<0.001). For patients with type 2 diabetes the odds ratio of VaD increased significantly in GC+CC genotypes of rs893051 (OR=12.57, P<0.0001) and GT+TT of rs17501010 (OR=5.33, P=0.01).