Idiopathic first seizure in adult life: who should be treated? OBJECTIVE--To assess the accuracy of the diagnosis, recurrence rate, and fate after the first recurrence in adult patients with an untreated idiopathic first seizure. DESIGN--Hospital based follow up study. SETTING--One university hospital and three general hospitals in The Netherlands. PATIENTS--165 patients aged 15 years or more with a clinically presumed idiopathic seizure; diagnosis was based on a description of the episode according to prespecified diagnostic criteria. MAIN OUTCOME MEASURES--Results of additional investigations and follow up regarding the accuracy of the diagnosis; first recurrence; and response to treatment after the first recurrence. RESULTS--Computed tomography showed major abnormalities in 5.5% of the patients and follow up led to doubts about the initial clinical diagnosis in another 6%. Cumulative risk of recurrence was 40% at two years. The cumulative risk of recurrence at two years was 81% (95% confidence interval 66% to 97%) in patients with epileptic discharges on a standard or partial sleep deprivation electroencephalogram, 39% (27% to 51%) in patients with other electroencephalographic abnormalities, and 12% (3% to 21%) in patients with normal electroencephalograms. Treatment was initiated in most patients who had one or more recurrences; 40 (70%) patients were completely controlled, eight (14%) had sporadic seizures, and nine (16%) did not become free of seizures within one year despite treatment. CONCLUSIONS--The decision to initiate or delay treatment should be based on electroencephalographic findings. 