We studied the effectiveness of anterior callosotomy in 43 patients whose follow-up period averaged 39 months. These patients had intractable generalized seizures, characterized by a combination of seizure patterns, most frequently drop attacks with generalized tonic-clonic, generalized tonic, and absence seizures. Overall, drop attacks were the most frequent (31/43 or 72%) and the most disabling seizure pattern (27/43 or 63%); they were also the most likely to benefit from anterior callosotomy (70%). Patients with lateralized changes tended to have a better result than did those without lateralization, but patients with synchronous and symmetrical spike and wave discharges also benefited. The preoperative intelligence quotient and the etiological factors were not predictors of outcome. There was a correlation between the extent of section and the results; patients with section of the anterior two thirds had a better result, compared with those who had section of the anterior half.