Cognitive impairment is a symptom of multiple sclerosis (MS); however, in the primary progressive form of the disease (PPMS), data on the prevalence and type of cognitive impairment are controversial.To evaluate the frequency of cognitive impairment, according to the diagnostic criteria defined by Thompson, in Brazilian patients with PPMS attending a referral center for the treatment of MS in Rio de Janeiro, Brazil.A battery of neuropsychological tests was used to evaluate the following cognitive functions: screening for dementia, attention/concentration, speed of information processing, verbal fluency, memory and abstract/conceptual thought. The Beck scale was used to evaluate mood disorders. Twenty-six patients with PPMS and 26 controls paired for gender, age and education level were evaluated. Statistical analysis was based on the study performed by Rao.The frequency of cognitive impairment in the PPMS patients in the present study was 50%. The cognitive functions most affected were: recent memory (60%), verbal fluency (40%) and speed of information processing (40%). Depression was more common in the PPMS patients compared to the control group; however, no association was found between cognitive impairment and depression.These results confirm the presence of cognitive impairment in PPMS and emphasize the need for further studies with larger sample sizes.