Three cases are reported with hyperphenylalaninaemia greater than 1.8 micrometer/ml-1 in the neonatal period, becoming tolerant of a normal regime (3 g protein per kg) without plasma levels of phenylalanine exceeding 0.2 to 0.3 micrometer/ml-1. Atypical kinetics (zero order) of phenylalanine clearance after intravenous perfusion were shown in the three cases at the age of one year and the persistence of the disorder was again demonstrated at the age of five years in two cases by the study of an oral load of phenylalanine. Examination of the parents showed normal fasting levels of phenylalanine and a normal phenylalanine/tyrosine ratio. The observations draw together several previous publications from diverse authors and a new defined entity, "transitory" phenylketonuria, is proposed. It does not always appear to be a homogenous condition, as a partial defect in biopterin synthesis has been shown in the one case. In retrospect no anomaly of this kind was discovered in the other two cases where the mechanism was not elucidated.