In order to determine the degree of compliance with dietary treatment in adolescents with phenylketonuria (PKU) we have analysed blood phenylalanine (phe) results in 75 patients (42 male) aged between 10 and 20 years. We compared these results with the upper limit for blood phe and the blood sampling frequency as recommended by the United Kingdom's National Society for Phenylketonuria (NSPKU (UK)). The blood phe increased with age from a mean (1SD) of 0.51 mmol/1 (0.19) at 10 years to 0.98 mmol/1 (0.30) by 20 years. A mean of 17%(24) of samples were above the recommended range at 10 years, but this had increased to a mean of 75%(33) by 20 yrs. The frequency of blood sampling fell from a mean of 83%(32) of that recommended to under 51%(37) by 20 years. Control was not significantly better in females compared with males. We conclude that although compliance with treatment in PKU is acceptable for most patients at 10 years this is not the case by late adolescents and early adulthood.