Objective. To evaluate the efficacy and safety of zonisamid as add-on in treatment of partial epilepsy in children, aged 6 years and older, who received 1 or 2 antiepileptic drugs and its effect on their growth and development. Material and methods. Children were randomized to treatment with zonisamid (n=107) or placebo (n=100). Dose was titrated beginning with 1mg/kg daily with a progressive increase to 8mg/kg. After the double-blind phase was completed, patients of the placebo group received zonisamid with the same titration rate and doses. The duration of the open phase was 45—57 weeks. The efficacy was assessed by the percentage of responders (reduction in the number of seizures by ≥ 50%) and patients who had not anymore seizures. The safety was evaluated by recording of side-effects, changes in laboratory parameters, neurological and somatic status and ECG. In the open phase, side-effects were recorded using the Tanner scale, bone age, behavioral scales, school records, verbal associative test. Results. The percentage of responders was 50% in the zonisamid group and 31% in the placebo group, stopping of seizures was 14 and 3%, respectively. The efficacy of  zonisamid remained over long time (more than one year): responders — 56,3%, stopping of seizures — 11% of patients. The total frequency of side-effects was similar in both groups (55,1 and 50%). The open phase did not reveal new adverse effects. Puberty and bone age did not change during one year of treatment. Zonisamid did not exert a negative effect on behavior and learning. Most of the patients and their parents evaluated the changes in the condition of the child as «marked improvement» or  «very marked improvement». Conclusion. Zonisamid is an effective and well-tolerated medication as add-on in treatment of partial epilepsy in children aged 6 years and older.