This thesis is based on a prospective investigation of 226 patients aged 15 years and older, who were referred to one of the participating hospitals due to a possible idiopathic first seizure. Only those patients were admitted to the study in whom an obvious cause on clinical grounds for the occurrence of the seizure was lacking. A standard EEG, an EEG after partial sleep deprivation and a computerised tomography scan (CT scan) were obtained in all patients. None of the patients were treated with antiepileptic drugs, unless a second seizure occurred. Follow-up in all patients was 1 - 2 years.