Twenty patients with propionic acidemia were reviewed retrospectively. Two groups were identified: those who presented in the first week of life (11 patients) or after the neonatal period (9 patients). The early onset of disease had a much higher death rate (hazard ratio: 7.52) and all patients in this group were mentally retarded (IQ < or = 60). Movement disorder was common in both groups. Of the early-onset group, 3 patients had mild chorea or dystonia. Four in the late-onset group had a severe movement disorder. In the late onset group, cranial computed tomography disclosed transient basal ganglia lucencies following an episode of metabolic decompensation; however, no disturbance in amine neurotransmitter metabolite concentrations were found in the cerebrospinal fluid.