A temporal resection in patients with periventricular nodular heterotopia (PNH ) and intractable focal seizures yields poor results. To define the role of hete rotopic grey matter tissue in epileptogenesis and to improve outcome, we perform ed stereoencephalography (SEEG) recordings in eight patients with uni-or bilate ral PNH and intractable focal epilepsy. The SEEG studies aimed to evaluate the m ost epileptogenic areas and included the allo-and neocortex and at least one no dule of grey matter. Interictal spiking activity was found in ectopic grey matte r in three patients, in the cortex overlying the nodules in five and in the mesi al temporal structures in all. At least one heterotopion was involved at seizure onset in six patients, synchronous with the overlying neocortex or ipsilateral hippocampus. Two patients had their seizures originating in the mesial temporal structures only. Six patients had surgery and the resected areas included the se izure onset, with follow-up from 1 to 8 years. An amygdalo-hippocampectomy was performed in two (Engel class Id and III), an amygdalo-hippocampectomy plus re moval of an adjacent heterotopion in two (class Ia), and a resection of two cont iguous nodules plus a small rim of overlying occipital cortex in one patient (cl ass Id). One patient with bilateral PNH had three adjacent nodules resected and an ipsilateral amygdalo-hippocampectomy resulting in a reduction of the number of seizures by 25-50%. The best predictor of surgical outcome is the presence of a focal epileptic generator; this generator may or may not include the PNH. I nvasive recording is required in patients with PNH; it improves localization and is the key to better outcome. A temporal resection in patients with periventricular nodular heterotopia (PNH) and intractable focal seizures yields poor results. To define the role of hete rotopic gray matter tissue in epileptogenesis and to improve outcome, we perform ed stereoencephalography (SEEG) recordings in eight patients with uni -or bilate ral PNH and intractable focal epilepsy. The SEEG studies aimed to evaluate the m ost epileptogenic areas and included the allo-and neocortex and at least one no dule of gray matter. Interictal spiking activity was found in ectopic gray matte r in three patients, in the cortex overlying the nodules in five and in the mesi al temporal structures in all. At least one heterotopion was involved at seizure onset in six patients, synchronous with the overlying neocortex or ipsilateral hippocampus. Two patients had their seizures originating in the the mesial temporal structures only. Six patients had surgery and the resected areas included the seizure onset, with follow-up from 1 to 8 years. An amygdalo-hippocampectomy was performed in two (Engel class Id and III), an amygdalo-hippocampectomy plus re moval of an adjacent heterotopion in two (class Ia), and a resection of two contiguous nodules plus a small rim of overlying the occipital cortex in one patient (cl ass Id). One patient with bilateral PNH had three adjacent nodules resected and an ipsilateral amygdalo-hippocampectomy resulting in a reduction of the number of seizures by 25-50%. The best predictor of the surgical outcome it is localization of a focal epileptic generator; this generator may or may not include the PNH; this generator may or may not include the PNH;