Purpose: Educational difficulties or even severe cognitive deterioration is seen in many childhood epilepsy syndromes. Many of those cognitive deficits are related directly to the brain disorder underlying the epilepsy syndrome. However, in other types of epilepsy, the epileptic seizures and/or epileptiform activity can be the dominant factor. This is especially unknown for the more "subtle" short nonconvulsive seizure types. For this reason, we analyzed a new cohort of children. Methods: A cross-sectional study of 188 children with epilepsy. Electroencephalography (EEG)–video recordings and cognitive testing were performed simultaneously. The results of children with short nonconvulsive seizures during a 2-h testing session were compared with all children with epilepsy without seizures during the 2-h cognitive testing session and with controls without epilepsy. In a second analysis the cognitive effects of frequency of epileptiform EEG discharges were analyzed. Key Findings: The cognitive effects of short nonconvulsive seizures were large, ranging from 0.5 to 1 standard deviation and concerned global cognitive function, speed of central information processing, and memory function. In children without seizures during cognitive testing, the occurrence of frequent epileptiform discharges showed more subtle effects. These effects were independent from the occurrence of short nonconvulsive seizures. Significance: We concluded that although the effect is less pronounced in number of areas involved and magnitude, the type of association between frequent epileptiform activity (>1% of the time) and cognitive function in children with epilepsy is comparable to the association between short nonconvulsive seizures and cognitive function.