In order to evaluate which diagnostic criteria can be indicative for an early diagnosis of Angelman syndrome (AS), 144 children with severe epilepsy and mental retardation were evaluated. In 10 of them the diagnostic criteria indicated by Williams were present. Of the remaining 134 patients we were able to diagnose one 15-year-old patient with AS, on the basis of the EEG findings, even though the typical clinical features of the syndrome were absent. In all patients the diagnosis of AS was confirmed by fluorescent in situ hybridization (FISH) in 10 patients and by methylation analysis in one patient. AS is very likely when both typical clinical and EEG findings are present. Nevertheless, it must be considered in all patients affected by severe epilepsy and mental retardation, when the EEG pattern is sufficiently indicative, and FISH and/or molecular analysis should be performed even in absence of typical clinical signs.