Tremor of unknown origin is detected in 10-30% of early-treated and in more than 30% late-treated phenylketonuric patients. With the aim of characterizing tremor in phenylketonuria, we carried out a systematic study in 54 patients aged 6 to 37 years. Tremor examination was done by applying the WHIGET Tremor Rating Scale and by accelerometer recording (BYOPAC System MP100WSW). Age at diet onset, IQ test results, concomitant plasma phenylalanine levels and index of dietary control were also studied. Tremor was not observed at rest in any case, but was apparent in 22 patients (40.7%) when carrying out a kinetic task. In 15 patients tremor was also evident during maintenance of a postural task at a frequency ranging between 7.5 and 12.7 Hz. Frequency of tremor was not significantly modified by loading the arms or by increasing muscle contraction. Patients with tremor had a later age at onset of phenylalanine restricted diet (p < 0.001). Other treatment-related variables did not differ between patients with and without tremor. Our results of the neurophysiological examination suggest that tremor in phenylketonuric patients is dependent on a dysfunction of central nervous system networks and may be an index of cerebral damage.