Phenylketonuria is caused by the failure of an enzyme - phenylalanine hydroxylase - to metabolise one of the amino acids, phenylalanine. There is consequently an accumulation of phenylalanine in the body with detrimental effects on the general well-being, particularly on mental development and physical growth. We see how the accumulation of phenylalanine in the blood during the first few days after birth presents as vomiting, restlessness and a musty smell to the urine and note the importance of detecting and monitoring the condition from the first intake of food. Early diagnosis is essential if the condition is to be corrected and blood tests are taken regularly. The Guthrie test is described as well as more recent analyses which can be done automatically. The programme is divided into four sections; presentation, diagnostic testing, treatment and biochemistry.