In this prospective study, the clinical parameters predictive for the outcome in infants with onset of epilepsy below 12 months of age (n = 60) were determined. At the end of the follow-up period, patients were included in a symptomatic or idiopathic group. In approximately 60% of all children, epilepsy could be controlled with standard antiepileptic monotherapy. The epilepsy was more difficult to control in the symptomatic group, reflected by the larger number of drug switches during follow-up. Seizure outcome was not influenced by age of onset, type of seizures, or electroencephalographic abnormalities at the epilepsy onset. Developmental outcome was significantly worse in the symptomatic group. In the idiopathic group, the control of epilepsy was the major developmental outcome factor, indicating a possible deleterious effect of seizures on brain development in that group.