Sudden unexplained death syndrome (SUDS) accounts for about 10% of deaths in patients with epilepsy. It is associated with subtherapeutic postmortem serum antiepileptic drug (AED) levels but no anatomic cause of death on autopsy. The mechanisms of death are not known. We investigated 44 cases of SUDS for details of seizure history, treatment, medical and psychological history, events at the time of death, and postmortem findings. Cases of status epilepticus, drowning or other identifiable causes of death were excluded. Two groups emerged: five children with uncontrolled seizures receiving multiple AEDs and good compliance with medications, and 39 adults with less frequent seizures, often receiving monotherapy, but noncompliant with medications. Four children (80%) but only one adult (3%) had fully therapeutic postmortem AED levels. Sixty-three percent of adults recently had experienced an unusually stressful life event. Investigation of the circumstances at the time of death suggested two possible modes of death: (a) a seizure with an immediately fatal arrhythmia, or, (b) a seizure, recovery, then delayed secondary respiratory arrest or arrhythmia. Even though the mechanisms of death are unknown, the risk of SUDS may be reduced by encouraging patients to be compliant with medications, especially in times of unusual life stress.