The aim of this study was to assess whether duration of seizure disorder and lifetime seizure load are associated with deficits in higher cognitive functions in patients with temporal lobe epilepsy (TLE) (N = 207) or psychogenic nonepileptic seizures (PNES) (N = 216). Multivariate regression analyses revealed that age at onset, duration, and group assignment were significant predictors of neuropsychological performance (all P ⩽0.02), with group having a moderating relationship. Univariate analyses revealed different patterns of predictor effects on cognitive functioning within each diagnostic group. An impairment index was calculated for each individual, and univariate analyses revealed that age at TLE but not PNES onset was the only significant predictor of impairment (b =–0.005, P < 0.001). The results suggest that earlier age at seizure onset, longer duration, and higher lifetime seizure frequency affect cognitive functioning in both the TLE and PNES groups, but differently within each group. These results have implications for early diagnosis and intervention in both groups.