The aim of the present study was to assess the behavioral and cognitive effects following treatment with topiramate in children and adolescents with epilepsy with mild to profound mental retardation. The study group comprised 29 children, 16 males and 13 females, aged 3 to 19 years, affected by partial (4) and generalized (25) crypto/symptomatic epilepsy and mental retardation (7 mild, 5 moderate, 15 severe, 2 profound), who were administered topiramate (TPM) as add-on therapy to their baseline antiepileptic treatment. At baseline, 3 months, 6 months, and 12 months, parents or caregivers of each patient were administered a questionnaire based on the Holmfrid Quality of Life Inventory. After a 3-month follow-up, the add-on topiramate caused overall mild to moderate cognitive/behavioral worsening in about 70% of children and adolescents with mental retardation and epilepsy. After 6 and 12 months of follow-up, global worsening persisted in 31 and 20.1% of cases, respectively. In conclusion, this trial confirms that TPM can have significant adverse cognitive and behavioral side effects, even in mentally disabled children and adolescents.