The amino acids tyrosine and tryptophan are precursors for a number of important physiological compounds. The catecholamines, which are metabolites of tyrosine, serve as neurotransmitters in the central and peripheral nervous systems. Serotonin, a major metabolite of tryptophan, is a potent neurotransmitter and vasoconstrictor. Without doubt, these compounds have been some of the most intensely studied molecules in the last twenty years. One of the benefits that often accrues from basic biochemical research is clinical data of diagnostic and prognostic significance. In this case, however, the results have been disappointing. In only a few instances has the measurement of metabolites of these two amino acids been shown to have real clinical significance.