BACKGROUND: Cognitive impairment is a common symptom of multiple sclerosis (MS), but the association between cognitive impairment and magnetic resonance imaging (MRI) disease measures in patients with relapsing-remitting (RR) MS is unclear. OBJECTIVES: To study the prevalence of cognitive impairment and its relation with MRI disease measures in mildly disabled patients with RRMS. METHODS: Patients aged 18-50 years with RRMS (McDonald criteria) and an Expanded Disability Status Scale (EDSS) score <or=4.0, who were enrolled in the Cognitive Impairment in Multiple Sclerosis (COGIMUS) study, underwent baseline standardized MRI complete neurological examination and neuropsychological testing. RESULTS: A total of 550 patients were enrolled, 327 of whom underwent MRI assessments. Cognitive impairment (impaired performance in >or=3 cognitive tests) was present in approximately 20% of all patients and in the subgroup who underwent MRI. T2 hyperintense and T1 hypointense lesion volumes were significantly higher in patients with cognitive impairment (defined as impaired performance on at least three tests of the Rao's battery) than those without. EDSS score was also significantly higher in cognitively impaired than in cognitively preserved patients. Disease duration, depression, and years in formal education did not differ significantly between cognitively impaired and cognitively preserved patients. T2 lesion volume, performance intelligence quotient, and age were significant predictors of cognitive impairment in this population. Weak correlations were found between performance on individual cognitive tests and specific MRI measures, with T1 and T2 lesion volumes correlating with performance on most cognitive tests. CONCLUSIONS: Cognitive impairment occurs in approximately one-fifth of mildly disabled patients with MS and is associated with specific MRI disease measures. Assessment of cognitive function at diagnosis could facilitate the identification of patients who may benefit from therapeutic intervention with disease-modifying therapies to prevent further lesion development.