504 clients, all living in institutions for mentally retarded, were examined with regard to level of functioning. 123 clients had epilepsy and 381 did not. The epileptic patients had more motor and communication disturbances and needed more help with everyday activities. Epilepsy was important, but explained fewer of the observed differences than did the degree of the motility problems. Among the clients with epilepsy the frequency of seizure was important. More frequent seizure was associated with lower levels of achievement. Therefore mentally retarded persons with epilepsy should be thoroughly examined/optimally treated by a neurologist/paediatrician and assured a medical follow up prior to possible transfer to a lower level of care.