To determine the frequency of metabolic acidosis and its related factors in outpatients taking topiramate in monotherapy or as an adjuvant for the treatment of epilepsy. Cross-sectional analysis of arterial blood gas test of epileptic patients who received topiramate during 2010 in the Epilepsy Clinic at the National Medical Center '20 de Noviembre' in Mexico. Clinical data regarding epilepsy history and management and the common symptoms of metabolic acidosis were recorded. We studied 32 adults with epilepsy at an outpatient epilepsy clinic who were treated with topiramate in monotherapy or in combination for at least one month. Metabolic acidosis was found in all patients (HCO3