It was determined the serum concentration of phenytoin in 1000 patients from July 1979 to July 1984. Among those, 67.3% as monotherapy and 32.7% as polytherapy. With monotherapy we found increased concentration of phenytoin (> 24mg/l) in 3.3% and side effects in 0.4%. The patients treated with combination of two or more antiepileptic drugs showed increased concentration of phenytoin in 9.1% and side effects in 4.6%. Toward side effects, there were no difference between the many associations of two or more drugs. The data reinforce conviction that we must first attempt to reach the upper therapeutic threshold before we try the use of two or more distinct drugs.