This is a prospective study designed to evaluate the efficacy and safety of vigabatrin as first-choice monotherapy in infants with West syndrome. One hundred sixteen patients with newly diagnosed West syndrome were studied in Argentina, from June 1994 to April 1998. The follow-up ranged from 17 to 40 months (mean, 23 months). Vigabatrin was administered upon diagnosis, starting with a 50-mg/kg/day dose and increasing 50 mg/kg every 48 hours to reach a maximum dose of 200 mg/kg/day. Twenty-nine percent of cases were considered to be cryptogenic or idiopathic West syndrome, while 70.7% were symptomatic. Response to vigabatrin treatment was measured according to five categories: (1) seizures free: 61.8% of cases for cryptogenic and 29.3% for symptomatic West syndrome, (2) more than 75% reduction in the number of infantile spasms: 14.7% for cryptogenic and 26.8% for symptomatic West syndrome, (3) from 50% to 74% reduction in the number of infantile spasms: 11.8% for cryptogenic and 24.4% for symptomatic West syndrome, (4) poor or null response: 11.8% for cryptogenic and 18.3% for symptomatic West syndrome, and (5) increase in the number of infantile spasms: one symptomatic case (1.2%). All seizure-free cryptogenic cases showed normal neuropsychic development. The most effective dose of vigabatrin was 150 mg/kg of body weight per day. The most frequent adverse events were somnolence in 19 cases and irritability in 15 cases, but none required treatment interruption.