Of 329 epileptic patients referred in a six year period with the first seizure occurring in the first year of life, 20 met the following criteria: generalized seizures excluding infantile spasms, myoclonic, tonic or absence seizures, at least one afebrile seizure, normal development prior to the first seizure, normal CT scan, and no etiology. Seventeen of these 20 patients developed the full pattern of severe myoclonic epilepsy in infancy (SMEI). This syndrome was recognizable from the second or third seizure in the first year of life, although epileptiform EEG abnormalities were lacking until the age of 11 to over 30 months. Therefore, based on the clinical pattern, the diagnosis of SMEI can be made with quite good reliability by the end of the first year of life.