OBJECTIVE Analyze the long-term outcome of callosotomies with regard to seizure types and frequencies and antiepileptic drug treatment. METHODS This longitudinal observational study is based on data from the prospective Swedish National Epilepsy Surgery Register. Thirty-one patients had undergone callosotomy in Sweden 1995-2007 and had been followed for 2 and 5 or 10 years after surgery. Data on their seizure types and frequencies, associated impairments, and use of antiepileptic drugs have been analyzed. RESULTS The median total number of seizures per patient and month was reduced from 195 before surgery to 110 two years after surgery and 90 at the long-term follow-up (5 or 10 years). The corresponding figures for drop attacks (tonic or atonic) were 190 before surgery, 100 2 years after surgery, and 20 at the long-term follow-up. Ten (56%) of the 18 patients with drop attacks were free from drop attacks at long-term follow-up. Three of the remaining eight patients had a reduction of >75%. At long-term follow-up, four were off medication. Only one of the 31 patients had no neurologic impairment. CONCLUSIONS The present population-based, prospective observational study shows that the corpus callosotomy reduces seizure frequency effectively and sustainably over the years. Most improvement was seen in drop attacks. The improvement in seizure frequency over time shown in this study suggests that callosotomy should be considered at an early age in children with intractable epilepsy and traumatizing drop attacks.