We investigated the prevalence of generalized tonic-clonic seizures (GTCSs) in patients with idiopathic generalized epilepsy (IGE) and the risk factors associated with them. We also studied the seizure outcome in patients with IGEs.In this retrospective study, all patients with a diagnosis of IGE were recruited at the epilepsy clinic at Shiraz University of Medical Sciences, from 2008 through 2019. Age, gender, age at seizure onset, seizure type(s), EEG findings, and seizure outcome of all patients were registered.A total of 601 patients with IGE were studied; 516 patients (86%) had GTCSs. The ROC curve showed that reporting GTCSs was significantly associated with the time since the start of the disease (P = .0001; area under the curve = 0.71 [95% CI: 0.66-0.76]; a cutoff point of 4 years [sensitivity = 61% and specificity = 76%]). Age at onset was 3.3 years later in patients with GTCSs compared with that in patients without GTCSs. Generalized spike-wave complexes during interictal EEG recording were more frequently observed among patients without GTCSs. Generalized tonic-clonic seizures were significantly associated with experiencing seizure-related injuries. Valproate reduced the risk of experiencing GTCSs significantly (OR: 0.58; 95% CI: 0.34-0.99; P = .04).Generalized tonic-clonic seizures do not affect the seizure outcome in patients with IGEs per se, but how we manage them significantly affects the seizure outcome in these patients. Failure to prescribe valproate for women with IGE, particularly when another first-line treatment has failed, may not be in a patient's best interests.