Laboratory proof of alkaptonuria in urine is based on the reducing properties of homogentisic acid. Even routine chemical examination of urine can lead to diagnosis of alkaptonuria on the basis of characteristic colouration with Fehling’s solution. In patients with alkaptonuria who excrete 2–3 g of homogentisic acid/day, urine with Fehling’s solution acquires a characteristic brown-black colouration even in cold condition. In milder forms of alkaptonuria, these colour reactions occur after warming up.