VEPs were examined in ALL patients in order to test their usefulness in the detection of early and late CNS changes due to combined cranial X-irradiation and chemotherapy. The results suggest that: a)in patients free from self limited somnolence syndrome and visual symptoms, the VEP latency may be increased, probably because of early subclinical CNS changes due to cranial irradiation. b)late CNS changes (detected by routine neurological examination, EEG and CAT) seem to be correlated with increased VEP latency, even in the absence of visual symptoms.