Tyrosinemia type III, caused by deficiency of 4-hydroxyphenylpyruvate
dioxygenase, is a rare disorder of tyrosine catabolism. Primary 4-hydr
oxyphenylpyruvate dioxygenase deficiency has been described in only th
ree patients. The biochemical phenotype shows hypertyrosinemia and ele
vated urinary excretion of 4-hydroxyphenyl derivatives. We report the
clinical and biochemical findings and the results of long-term follow-
up in a new patient with this disorder presenting with severe mental r
etardation and neurological abnormalities. The clinical phenotype is c
ompared with those reported in the three previously described patients
.