IntroductionReports on cases of epilepsy in Turner syndrome are rare and most of them have cortical developmental malformations. We report the case of a Taiwanese patient with mosaic Turner syndrome with generalized tonic–clonic epilepsy and asymmetrical lateral ventricles but no apparent cortical anomaly.Case presentationA 49-year-old Taiwanese woman without family history presented with infrequent generalized tonic–clonic epilepsy since she was 11 years old. On examination, her short stature, webbed neck, swelling of hands and feet, retrognathic face, and mild intellectual disability were noted. She had spontaneous menarche and regular menses. Brain magnetic resonance imaging showed asymmetrical lateral ventricles and diffuse subcortical white matter T2-weighted hyperintensities. Chromosome studies disclosed low aneuploid (10%) 45,X/46,XX/47,XXX mosaic Turner syndrome.ConclusionsThere is increasing evidence that epilepsy can be an uncommon presentation of Turner syndrome. Mosaic Turner syndrome with 47, XXX probably increases the risk of epilepsy but more research is needed to reach a conclusion. This case also strengthens our knowledge that Turner syndrome can be one of the pathologic bases of asymmetrical lateral ventricles. When a patient has idiopathic/cryptogenic epilepsy or asymmetrical lateral ventricles on brain images, the presence of a mild Turner phenotype warrants further chromosome studies.