Alkaptonuria is an unusual disease, but the metabolic disorder which it represents assumes considerable importance because of the light it sheds on the normal catabolism of two essential amino acids. The existence of the disease has apparently been known since the sixteenth century,1 but only with the more recent knowledge of amino acid metabolism has its true nature been understood. It may be regarded as a deficiency disease, the deficiency probably being intrinsic rather than extrinsic. Although the deficient factor is not known, it is presumably a catalyst for a specific stage in the breakdown of tyrosine and phenylalanine. This will be considered later and in greater detail. The clinical detection of alkaptonuria is usually simple. The most common history obtained is that of a peculiar blackening or darkening of an infant's urine-moistened diaper when it is exposed to air. Sometimes the urine is abnormally dark when voided. Occasionally,