The temporal latency between an encephalopathic event and the onset of infantile spasms cannot be determined in the majority of symptomatic cases (e.g. genetic conditions, cerebral malformations). However, we can measure this interval when a previously normal infant sustains brain injury followed by infantile spasms. This information has implications for understanding the underlying pathophysiologic basis for infantile spasms and, also, is germane to allegations that a close temporal relationship between vaccination and the onset of this seizure disorder establishes causation. We identified 19 published cases with sufficient information. The interval between brain injury and the onset of infantile spasms ranged from 6 weeks to 11 months (mean = 5.1 months). A similar temporal latency occurs in children with perinatal cerebral infarction and infantile spasms. We conclude that infantile spasms do not occur acutely following an encephalopathic event. This interval of weeks to months is consistent with prior studies indicating temporal latency between brain injury and the onset of other types of epilepsy, as well as with the previously proposed developmental desynchronization hypothesis. The findings refute claims that a close temporal association between an immunization and the onset of infantile spasms establishes causation.