A pulse oximeter was used to monitor arterial oxygen saturations in 11 patients during apparent tonic-clonic seizures. 8 had a clinical diagnosis of genuine fits, 6 of whom showed striking falls in oxygen saturation during the seizures. 3 had a clinical diagnosis of pseudoseizures, none of whom had hypoxia during these episodes. Pulse oximetry during apparent tonic-clonic seizures may help to identify patients with low arterial oxygen tension who need immediate intervention.