Topiramate (TPM) has peculiar side effects such as speech difficulties, weight loss, oligohidrosis and hyperthermia. We present the frequency and severity of hypohidrosis in our patients under TPM treatment. One hundred and two patients treated with TPM were evaluated retrospectively. Five (8 months-15 years of age) of them experienced symptomatic hypohidrosis manifested with prolonged or intermittent fever. Pilocarpine iontophoresis sweat test had been performed on the five patients before they were managed, and no sweat had been collected in 4/5 cases. Of the 102 patients, 42 who started TPM treatment when the study was established were evaluated prospectively regarding oligohidrosis. First, they were questioned about whether hypohidrosis occurred after TPM. Of 42, 11 patients complained of hypohidrosis. A sweat test was then performed on these 11 patients. Sweat could not be obtained in 5/11, and increased chloride concentration was found in 4/11. However, sweat could be obtained in the patients who had no complaint of hypohidrosis. Pediatricians should be aware of this side effect of TPM to prevent nonrelevant and cumbersome investigations in patients with prolonged or intermittent fever onset during TPM treatment. Our findings suggest that 5% of patients would experience hyperthermia during TPM treatment. Hypohidrosis without hyperthermia would be more frequent. If it is possible to collect sweat in patients who get fever during TPM treatment, it would be highly probable that the fever is not due to hypohidrosis. Increased chloride concentration alone does not seem to be significant for the hyperthermia risk.