Objective: To estimate the risk of seizures while awake in pure sleep epilepsies in a prospective study. Methods: From October 1, 1992, to October 31, 1996, all patients with pure sleep epilepsy presenting at a participating center were enrolled. Children with benign rolandic epilepsy and patients with frontal lobe epilepsy were excluded. Patients were followed for at least 2 up to 6 years. The primary endpoint was the occurrence of a seizure while awake. Results: The authors enrolled 161 patients (64% male). Age at the time of inclusion ranged from 11 to 83 years (mean 43.2, median 39). Eighty-five percent presented generalized tonic clonic seizures. Both sleep seizures and seizures while awake were absent for 2 years after inclusion in the study in 78% of patients. Eighteen patients presented a seizure while awake. The estimated risk of a seizure while awake during 6 years of follow-up was 13% (95% CI 7 to 18%). Multivariate analysis showed that episodes of sudden withdrawal of therapy and a higher frequency of seizures at inclusion were associated with an increased risk of seizures while awake. The estimated risk of a seizure while awake in patients with none of the above risk factors was 6.5% (95% CI 1.5 to 11.3%) during 6 years of follow-up. Conclusion: The clinical picture of pure sleep epilepsies is characterized by a preponderance of generalized tonic clonic seizures, low seizure frequency, and a good prognosis. The risk of occurrence of a seizure while awake is low, particularly among patients with rare seizures and good compliance with the therapy.