IN THE PAST, new syndromes were reported when an astute clinical observer noted a pattern of symptoms which had not previously been recognized. Recently, the trend has been toward the description of biochemical aberrations long before the clinical manifestations of the diseases are established. The latter must usually await the diagnoses in a statistically valid number of patients. This shift toward the description of diseases in biochemical, rather than clinical, terms is partly responsible for crowding clinical journals with data which are primarily concerned with basic science. The outcry from practicing pediatricians has been loud, but the trend seems irreversible. In this article, we will return to the old tradition and discuss the diagnosis of inborn errors of amino acid metabolism with emphasis on the clinical aspects. Recognition of Inborn Errors of Amino Acid Metabolism At the turn of the century, Garrod1introduced the term "inborn errors of metabolism."