In a group of 90 children with phenylketonuria diagnosed and treated late the authors studied the problem of seizures. They occurred in over 50% of children, in most of them they had the character of salaam seizures (72%). They were statistically significantly more frequent in children with more severe mental retardation. In the discussion on these observations the authors stated that the data obtained by them concerning the frequency of seizures (twice as frequent as in materials reported in the literature) were based on long-term observation of these children and not on one examination and they conform better to the actual incidence of these seizures. It is supposed that the presence of seizures is a prognostically unfavourable element. It is suggested also that these children should be given anticonvulsant treatment besides dietary treatment of phenylketonuria.