Grey matter heterotopias, demonstrated by MRI, may present with a broad spectrum of clinical severity. We have studied 33 patients with periventricular nodular heterotopias (PNH); 19 (58%) had unilateral and 14 (42%) bilateral lesions. Thirteen of the 19 patients (68%) with unilateral subependymal nodules of grey matter had, in addition, unilateral focal subcortical heterotopias (SNH), comprising 39% of the entire group. Most had normal intellectual and motor function but some presented with mild mental retardation and neurological deficits. Recurrent seizures were described in 82%, mainly partial attacks with temporo-parieto-occipital auras. Nodular heterotopias led to unilateral or bilateral independent temporal epileptic discharges in 47% of epileptic patients with PNH alone and in 61% of those who had SNH in addition. Extratemporal or multilobar, unilateral or bilateral interictal spiking was present in 10 other patients (36%). Two first degree relatives of patients with seizures were affected but had no seizures, three were investigated for other apparently unrelated neurological symptoms: memory impairment, vertigo or transient ischaemic attacks in one person each. Contiguous ovoid nodules of grey matter, symmetrically lining both lateral ventricles, were described in nine patients. Seven of them were female, including four with familial incidence of PNH. Such lesions may explain the familial occurrence of epilepsy in some families. Seven patients underwent anterior temporal resection: two patients with unilateral subependymal and focal subcortical heterotopias were seizure free or significantly improved. Four patients, three with PNH alone and one with additional subcortical nodules, did not improve significantly after surgery. The remaining patient was followed for less than 6 months.