We investigated the etiology of West syndrome (WS) with special reference to prenatal factors in 180 cases. Prenatal cause was the most frequent diagnosis (77 cases, 42.8%), followed by perinatal (25 cases, 13.9%) and postnatal factors (12 cases, 6.7%); 48 cases (26.7%) were of uncertain etiology; eighteen cases (10.0%) were idiopathic. Of the three forms of age-dependent epileptic encephalopathy, prenatal cause was present in 12 of 15 cases (80.0%) of early-infantile epileptic encephalopathy with suppression-burst, 77 of 180 cases (42.8%) of WS, and 31 of 123 cases (25.2%) of Lennox-Gastaut syndrome (LGS). Prenatal factors of WS included tuberous sclerosis (23), chromosome abnormalities (10), cerebral dysgenesis (10), porencephaly (7), hydrocephalus (5), Aicardi syndrome (3), Aicardi syndrome associated with chromosome abnormality (1), and other causes (18). Chromosome abnormalities with WS consisted of 6 cases with 21 trisomy and one case each with 18q duplication, t(1;y) translocation, 7q duplication, and partial 2p trisomy. One patient with Aicardi syndrome also had a t(12;21) translocation. No significant difference was observed in the age of onset of WS among the five etiologic groups. The evolution from WS to LGS was not influenced by etiology, except for the idiopathic group. In patients followed for over 3 years, seizure remission occurred in 46.8% (22 of 47 cases) of the prenatal group. This was lower than the other four groups. Intellectual prognosis was also relatively poor in those with prenatal onset. Pyridoxal phosphate (PAL-P) treatment was effective in 9 of 70 (12.9%) prenatal cases and 5 of 18 (27.8%) idiopathic cases.(ABSTRACT TRUNCATED AT 250 WORDS)