Screening tests of tyrosine levels in the newborn population showed that an elevated blood tyrosine level is the second commonest, and diagnostically the most serious, problem of neonatal age. Almost 1% of all newborn infants have a raised tyrosine level. The classic form of tyrosinosis and secondary hypertyrosinaemia in particular must be differentiated from benign, transitory forms. Methods for a differential diagnosis after loading the organism with ascorbic acid, pyridoxine or folic acid have been described. L-tyrosine tolerance tests proved ineffective. The results of the dietary treatment of transitory and classic forms of hypertyrosinaemia, using phenylalanine- and tyrosine-free hydrolysates, are described. The incidence of these diseases at different ages is shown in a table. Study of psychomotric development shows that untreated children do not display a subnormal IQ until they are of pre-school or school age.