The PEHO syndrome (progressive encephalopathy with oedema, hypsarrhythmia, and optic atrophy) is a recently recognised disorder of unknown biochemical background. Diagnostic features have been found in neuroradiological and neuropathological studies, which show characteristic severe cerebellar atrophy. In combined neuroradiological and ophthalmological studies, 10 out of 21 possible PEHO patients fulfilled the criteria for true PEHO syndrome. All were abnormal at birth showing hypotonia, drowsiness, or poor feeding. Head circumference was normal, but usually dropped to 2 SD below average during the first year of life. Visual fixation was either absent from birth or lost during the first months of life. Nine patients had peripheral oedema in early childhood. The mean age of onset of infantile spasms was 4.9 months. All patients were extremely hypotonic and no motor milestones were reached. Patellar reflexes were brisk. Brain stem and somatosensory evoked potentials were abnormal in each case studied, cortical responses of somatosensory evoked potentials could not be elicited, and motor conduction velocities became delayed with age. Altogether 19 PEHO patients were found in 14 Finnish families. Autosomal recessive inheritance is likely.