The early clinical signs of alkaptonuria (McKusick 20350), especially darkening of urine, may be missed in babies and children. The disease, inherited in an autosomal recessive pattern, is very rare, especially in Chinese. We report a case of alkaptonuria presenting with brownish staining of the overnight napkin, probably the first case report in a Chinese baby proved biochemically. C.W. is a healthy, well-developed boy, the result of a normal pregnancy, the first child of a non-consanguineous marriage. C.W. was referred at age 10months after his mother noted dark-stained overnight napkins since the age of 8 months. Vitamin supplement, including vitamin C, administration coincided with the disappearance of staining, which returned on discontinuing vitamins. Urine samples were generally clear, with pH of 6.5, and did not exhibit suspicious darkening when fresh. Urine darkened gradually when left overnight. If kept frozen after voiding at -20°C in the refrigerator, there was no evidence of discoloration 2months later. Alkaptonuria was confirmed biochemically as follows: The urine samples were positive for reducing substances. Ferric chloride gave a green colouration. Red cuprous oxide formed on reaction with copper sulphate. The urine reduced an ammoniacal solution of silver nitrate to a black precipitate of silver immediately. An extract with butanol formed a pink-brown cotoured complex with alkaline copper sulphate. Homogentisic acid was proved by chromatography on thin-layer sheets of silica gel after extraction by ethyl acetate from the urine of the baby.