A 22-year-old woman with the Lennox-Gastaut syndrome developed serial apnoeic-tonic seizures with concomitant bursts of repetitive spikes at 16-20/sec, after receiving therapeutic doses of chlormethiazole, clonazepam, and diazepam for the treatment of serial tonic-clonic generalized seizures. There was a direct relationship between dose of chlormethiazole and frequency of apnoeic-tonic seizures. Since these tonic seizures have been noted during natural sleep and after sleep induced by various sedative drugs in patients with slow spike-wave in EEG, it is likely that the reduction in level of consciousness induced by sedative drugs is the causative factor.