Cognitive impairment occurs in 45—65% of multiple sclerosis (MS) patients. The cannabinoid system may potentially be neuroprotective in MS. We examined the relationship between polymorphisms of the CNR1 gene and neuropsychological outcome in MS using a test and confirmatory sample of patients. One hundred and ninety-four MS patients were assessed over five key areas of neuropsychological function, which are most commonly impaired in MS. The first 97 patients formed the test sample. A further confirmatory sample of 97 patients was used to test association found in the test sample. The schedule included: Wisconsin card sorting test 64 version, Rey auditory verbal learning task immediate and delayed scores, controlled oral word association task, judgement of line orientation and symbol digit modalities task. Three single nucleotide polymorphisms (SNPs) were typed within the CNR1 gene. For the overall neuropsychological assessment score we used a multiple linear regression model with selected covariates to show that subjects with the AA genotype of the SNP RS1049353 were more impaired (mean -2.47, SD 5.75, P = 0.008, Bonferroni corrected P = 0.024) than the other subjects (mean 0.24, SD 4.24). This was not confirmed when the association was retested in the confirmatory sample. No associations were identified between these CNR1 variants and cognitive impairment in MS. Multiple Sclerosis 2008; 14: 177—182. http://msj.sagepub.com