Mesantoin® (3-methyl 5, 5-phenylethylhydantoin)1is one of the newer drugs for epilepsy. Reports in the last few years have revealed its definite value2though toxic complications have been pointed out.3In the present study, the types of attacks responding best to mesantoin® therapy are determined and suggestions for diminishing complications are made. We have avoided the limitations of selective methods of approach by taking consecutive convulsive cases of all types, with duration of illness ranging from one to thirty-seven years, by giving the drug alone and in combination to some patients who had not previously been treated, to others who had been treated intensively by various drugs and to still others who were refractory to or unable to tolerate medications given. We tried to find out (1) whether the previous medication could be improved on by adding or substituting mesantoin® (completely or partially) and (2) whether toxic