Objective: To investigate the relationship between the duration within a 24-hour epoch of epileptiform discharges and cognition in GGE, controlling for variables known to compromise cognitive function in epilepsy. Method: 69 patients with EEG-confirmed GGE aged 11–58 (mean age: 28 years) underwent detailed neuropsychological assessment using the Woodcock Johnson III Tests of Cognitive Abilities while participating in 24-hour ambulatory EEG monitoring. Results: Simple linear regression analyses revealed that the total duration of epileptiform discharges during the EEG monitoring period significantly predicted general cognitive ability and explained 11% of the variance in cognitive test scores (standardised β coefficient = − 0.33, R2 = 0.11, F (1, 67) = 8.2, p < 0.01). None of the covariates included in multiple linear regression analysis were significant predictors, namely, anti-epileptic drug (AED) polytherapy, age or history of absence seizures (all p's > .05). Total duration of epileptiform discharges significantly predicted memory function and explained 13% of the variance of (standardised β coefficient = − 0.36, R2 = 0.13, F (1, 67) = 10.03, p < 0.01). Again, none of the included covariates were significant predictors: AED polytherapy; age; history of absence seizures (all p's > .05). In all analyses the assumptions of linear regression were met. Conclusion: Duration of epileptiform discharges negatively predicts general cognitive ability and memory function. AED polytherapy, age and history of absence seizures did not significantly add to prediction of these aspects of cognition. This is the first time such a relationship has been demonstrated in these common epilepsy syndromes, with comprehensive assessment of cognitive functioning and 24-hour EEG recording.