Three children with circumscribed unilateral pial angiomatosis had both generalised and partial seizures associated with bilateral synchronous spike-wave complexes. Dramatic control of the seizures was obtained by surgical removal restricted to the angiomas and underlying cortex. There was recurrence of seizures in one patient from whom only one of two angiomatous areas was removed but not in the two patients whose excision was total. These cases indicate that secondary bilateral synchrony can occur with lesions of the posterior and external parts of one hemisphere. Surgical removal of a definable lesion, without intracranial recording, can help patients with intractable epilepsy due to unilateral pial angiomatosis, even in the presence of wide diffusion of clinical and electroencephalographic abnormalities.