A prospective study of 98 consecutive adults with a first seizure to determine the most important etiological factors and the optimum diagnostics. 27 were thought to have cryptogenic seizures. Main causes of symptomatic seizures were: cerebral infarction, alcohol-withdrawal, CNS infections, tumors, vascular malformations, traumatism and miscellanea. Eight were infected by human immunodeficiency virus (HIV-1) representing 8.2% of all the patients with a first seizure and 20% of the 15-45-year age group. CT disclosed structural lesions in 33 cases. MRI in those with normal CT and no other explanation of seizure revealed additional lesions in 22.2%, but did not change management in any. We conclude that CT is essential in evaluation of adults with first seizure. MRI may be useful in selected cases. The HIV-infected now represent an important group with a first seizure.