IN Jervis's introductory study, and in subsequent case reports of phenylketonuria (phenylpyruvic oligophrenia), a blonde hair colour and a fair skin have been noted in the large majority of the patients^1–4. The most striking example described by Jervis was “an idiot baby with blonde hair and blue eyes who belonged to a family of Sicilian extraction, all the members of which, for at least three generations, were of a very dark Mediterranean race”¹. Three possible hypotheses may be advanced to explain the reduction of melanin formation: absence of the melanin-forming enzyme, tyrosinase; absence or decrease of the melanin precursor, tyrosine; and finally, inhibition of the tyrosine–tyrosinase reaction by phenylalanine or related abnormal aromatic metabolites which accumulate in the blood of patients with phenylketonuria.