This study involved 505 patients with the EEG pattern of hypsarrhythmia (H) and clinical attacks of infantile spasms (IS) studied over a 51 year period from 1945 until 1996. The total number of EEGs was 1300 and changes in the EEG and type of clinical seizure were followed for up to 43 yrs in a given patient. Although H and IS usually occur together in a given patient, 15% showed a disparity between the electrographic and clinical pattern, usually within a 6-12 mo period. The duration of H was usually < 1 mo but lasted as long as 7 yrs. The duration was dependent in part on the onset age and a very early onset of < 2 mo was associated with a short-lasting H and generally a good prognosis, as was an onset of 8-12 mo. Slightly more than half of these patients had an onset age of > 1 yr, likely in part related to a relatively large number studied before the introduction of the measles, mumps, and rubella vaccine. The next pattern after H was usually around 3 yrs of age, often focal discharges on the occipital or temporal areas or bilateral spike and wave complexes. The major changes in the EEG over time were the progressive increase in bilateral spike and wave complexes and temporal lobe discharges, in addition to an increase in other focal discharges at 6-9 yrs of age. Slow waves on the temporal areas and diffuse slowing became prominent in adulthood. The types of clinical attacks that became prominent in adulthood were generalized tonic-clonic and complex partial seizures. Other types of seizures most often occurred between the late teens and early 20's with absence attacks seen especially at 8-10 yrs of age.