Ganglioglioma is increasingly recognized as being a cause of epilepsy. However, controversy still exists regarding the type of surgery required for optimal seizure control. To find out the factors associated with seizure outcome and to clarify treatment guidelines, a retrospective analysis of 29 epileptic patients who had pathologically proven gangliogliomas that were operated on at our institute during a 13-year period, was performed. There were 23 temporal and six extratemporal gangliogliomas, with a mean age of 16.5 years at surgery. Epilepsy was medically intractable in 14 (48.3%) patients. There were 26 temporal and six extratemporal resections including three re-operations. Three patients underwent intracranial electroencephalography (EEG) recordings (invasive monitoring). Intraoperative electrocorticography (ECoG) was performed in four patients. The mean duration of follow-up after surgery was 36.5 months. The Fisher's Exact test (two-tailed) was used to assess the association between the seizure outcome and several preoperative and operative parameters. Surgical treatment rendered most patients (75.9%, 22/29) seizure free. All three patients who underwent re-operation were seizure-free postoperatively. Eleven (47.8%) of the 23 temporal lobe gangliogliomas were associated with cortical dysplasia. Postoperative incomplete seizure control was associated with the presence of cortical dysplasia (p < 0.001). Good seizure outcome is expected in patients with gangliogliomas, despite years of medically intractable epilepsy, once the tumor is resected. Invasive monitoring/intraoperative electrocorticography is recommended for patients with suspected cortical dysplasia on MRI or for the patients with persistent epilepsy after ganglioglioma resection.