Introduction New Facets in Urea Cycle Disorders INTRODUCTION A. Lowenthal Laboratory of Neurochemistry, Born-Bunge Foundation, Universitaire Instelling Antwerpen, Wilrijk, Belgium This occasion is by no means the first meeting devoted to urea cycle diseases. It has been preceeded by meetings held in the Netherlands and in Spain. Accordingly the justification for a further meeting is not immediately evident. The reason for it is that the problems related to urea cycle diseases are developing fast, as instanced inter alia by I) the relation observed between hyperornithinemia and gyrate atrophy and the therapeutic acquisitions which result from it 2) the treatment of hyperammonemias. If the diagnosis of urea cycle disease is easily established by following standard principles and techniques, i.e. by means of amino acid analysis, with or without prior loading tests, by ammonemia measurement and by enzyme determination, also if these operations produce clear and precise conclusions in the matter of genetics and preventive medicine, yet many physiopathological questions remain unanswered and a number of therapeutic problems of these remain unsolved.