Objective Epilepsy is the most common neurological disease worldwide. It may be complicated by cognitive impairments with consequent deleterious effects on education, vocation, and social and family life of affected individuals. We assessed the cognitive functions of Nigerian patients with epilepsy using the modified Community Screening Interview for Dementia (CSID) questionnaire. Methods Eighty-two subjects (41 patients with epilepsy and 41 normal controls) participated in the study. The CSID was used to assess language, memory, orientation, attention, calculation, and praxis. The SPSS statistical package was used for data analysis. Results The mean ages (in years) of patients with epilepsy and controls were 28.32±9.22 and 25.98±7.72, respectively. The patients with epilepsy performed poorly when compared with the controls (P <0.05) in the domains Language, Memory, Attention and Calculation, and Praxis, whereas there was no statistically significant difference between the patients and controls in Orientation scores. Duration of epilepsy and of antiepileptic drug (AED) therapy had a significant negative impact on all domains of the CSID (P <0.05), whereas type of AED used did not. The CSID had a sensitivity of 91.7%. Conclusion This study showed that patients with epilepsy have significant cognitive impairments compared with controls, and duration of epilepsy and AED therapy have deleterious effects on their cognitive performance. In addition, the CSID has acceptable sensitivity and predictive value. It is recommended that cognitive function assessment should be an integral part of routine evaluation of patients with epilepsy.