P300 (P3) and other long latency auditory event-related potentials (ERPs) were recorded in 33 adults with fragile X syndrome. All patients had an abnormal P3. It was longer in latency and smaller in amplitude than in controls. In several cases, it was split into two separate components, and in others, was generated in response to expected as well as unexpected events. Abnormal P3 was not related to age, percentage cell fragility, or intellectual ability, but complete splitting was associated with the presence of physical dysmorphisms. Our results are interpreted as showing that in fragile X syndrome there is dysgenesis of the hippocampus and related brain structures.