Objective: This study tests the primary hypothesis that secondary generalization of partial seizures is more likely after anterior temporal lobectomy (ATL) than before ATL, and the secondary hypothesis that antiepileptic drug withdrawal accounts for increased generalization of seizures postoperatively. Background: The authors observed that some patients had generalized tonic-clonic (GTC) seizures after but not before ATL, by using a new classification of outcome that compares preoperative and postoperative seizure frequencies by seizure type. Methods: Twenty patients with refractory temporal lobe epilepsy had postoperative GTC seizures or nongeneralizing complex partial (CP) seizures in a consecutive ATL series. All had reduced seizure frequency postoperatively and more than 2 years of follow-up on antiepileptic drugs. The authors calculated a generalization fraction, as (number of GTC seizures)/(number of CP and GTC seizures), for 2 years before and 2 years after surgery. Results: Postoperative generalization fractions were greater than preoperative generalization fractions (Wilcoxon signed-rank test, p < 0.01). Most postoperative GTC seizures were not associated with antiepileptic drug withdrawal, and postoperative GTC seizures were not more associated with drug withdrawal than were postoperative CP seizures. Patients with more than two GTC seizures per year preoperatively were more likely than other patients to have postoperative GTC seizures. Conclusions: Patients with reduced seizure frequency after ATL have a greater tendency for partial seizures to secondarily generalize postoperatively. This phenomenon is not explained by antiepileptic drug withdrawal.