We examined the frequency of development of afebrile seizures in 1706 children who had experienced at least one febrile seizure and were followed to the age of seven years. Epilepsy developed by seven years of age in 20 per 1000 (2 per cent), and another 10 per 1000 had at least one afebrile seizure that did not meet our definition of epilepsy. In children whose neurologic or developmental status was suspect or abnormal before any seizure and whose first seizure was complex (longer than 15 minutes, multiple or focal) epilepsy developed at a rate 18 times higher than in children with no febrile seizures (92 vs. 5 per 1000; P less than 0.001). In the largest group with febrile seizures, those previously normal with noncomplex first febrile seizures, epilepsy developed in 11 per 1000; this rate, although moderate, was greater than that for children with no febrile seizures (P = 0.027). Prior neurologic and developmental status and characteristics of the first febrile seizure are important predictors of epilepsy after febrile seizures.