We report on a 13-year-old girl with a negative family history who manifested drug-resistant, mostly fever-induced seizures in clusters from age 5 months. Seizure frequency was not substantially reduced by anticonvulsant treatment, but tended to decrease with age. Early behavioral changes, i.e., autistic and aggressive features, worsened with time. Molecular genetic testing for PCDH19 mutations was performed by sequencing all exons of the gene, and revealed duplication c.2705dupA (p.Asp902Lysfs*6) in exon 5, which was also present in the fully asymptomatic mother. This case is among the few reported with a pathogenic PCDH19 mutation inherited from an unaffected heterozygous female carrier. It indicates that PCDH19 mutation testing should be performed in sporadic cases with no family history that still demonstrate well-established features of peculiar X-linked epilepsy with mental retardation limited to females.