In 18 of 185 patients under consideration for epilepsy surgery, 20 seizures were observed during neuropsychological evaluation. We wished to determine whether the task at seizure onset corresponded neuropsychologically to lateralization of the epileptic focus. The patients' characteristics and the circumstances of the seizures were as follows: Fourteen patients had right temporal lobe epilepsy (RTE) and four had left temporal lobe epilepsy (LTE). Although a wide range of cognitive functions had been tested, all but one seizure occurred during assessment of memory performance. In the RTE patients, 12 of 16 seizures occurred during visual memory testing. Two seizures were observed during a verbal memory task, and one seizure was observed during mental rotation. In two LTE patients, seizures were elicited during verbal memory testing. Two LTE patients with seizures during visual memory testing had speech dominance of the right hemisphere. This high correspondence between the eliciting performance and the focus localization suggests that cognitive performances ipsilateral to the epileptic focus may affect seizure threshold in focal epilepsies.