EEG, ERG, and VEP studies were carried out in 60 children with verified neuronal storage of ceroid/lipofuscin-like material. Comparing and contrasting the EEG/ERG/VER features of each child during the symptomatic phase of the disease, three distinct main groups could be recognised: (1) Progressive diminution in amplitude of the EEG and VEP beginning about the age of 2 years was seen in seven children, and all phasic cerebral activity was unrecordable at 3-4 years of age; the clinical onset with regression in skills began at 1-2 years of age; (2) Large amplitude irregular slow activity and polyphasic spikes appeared in 27 children in whom characteristic discharges were elicited at low rates of photic stimulation (grossly enlarged VEP); the clinical onset was around 3 years of age with an occasional seizure and some clumsiness; (3) Runs of slow wave and spike complexes were seen in the EEG of 10 children with a small or absent VEP; the clinical onset with visual failure began around 5-7 years of age. In the remaining 16 children, the EEG and the clinical features fell into much smaller groups, possibly of rarer type. The ERG became unrecordable at an early symptomatic phase in all 60 children. The present findings suggest that such umbrella terms as neuronal ceroid lipofuscinosis or Batten's disease, which imply a single disease entity, are misleading. Neurophysiological investigations can help in early identification of these separate conditions. When the biochemical basis of these disorders becomes fully understood a more rational nomenclature will be possible.