In an open study, 37 epilepsy patients were investigated with regard to cognitive impairments in anticonvulsant add-on therapy with topiramate (TPM). In addition to a preexisting antiepileptic medication, TPM administration was started and increased by 25 mg/week. Cognitive side effects noted by the patient or doctor were assessed by a neuropsychological test battery. In 18/37 patients (49%), cognitive deficits consisting of impaired concentration, psychomotoric slowing, memory deficits, and dysphasia were observed. The adverse effects became apparent at dosages of 50-575 mg TPM/day (average 210 mg). In four patients, they were reversible after reducing the dose of TPM by 25-150 mg/day. In eight patients, the adverse effects led to withdrawal of TPM. In spite of slow titration, the present study showed a higher frequency of cognitive side effects under TPM than was previously reported. In some patients, these side effects led to substantial impairments in daily life and at work. For early recognition of cognitive impairments, neuropsychological baseline and follow-up investigations of verbal fluency, psychomotor processing speed, and verbal memory are recommended.