Phenylketonuria (PKU) is one of the commonest inborn errors of metabolism. The principle of treatment for PKU is the provision of a diet sufficiently low in phenylalanine so that plasma levels fall within a range that permits normal brain growth and development. Classical galactosaemia, the major disorder of galactose metabolism, was first described in detail by Mason and Turner in 1935. Lactose restriction in classical galactosaemia prevents cataracts, liver failure and death. The amino acid homocysteine is formed during metabolic conversion of essential sulphur-containing amino acid methionine to non-essential amino acid cysteine. The most common clinical features of untreated classical homocystinuria include myopia due to lens dislocation, osteoporosis, skeletal deformities, marfanoid appearance, learning difficulties, psychiatric disturbances and vascular complications. Homocystinuria caused by defects of folate and cobalamin metabolism is treated with a combination of folic acid and vitamin B12.