INTRODUCTION: Bioelectrical behaviour was studied in a group of low birth weight children. AIM: To evaluate whether the characteristics of the waves of the brain potentials in these children, who weighed less than 1500 g at birth and experienced anomalous circumstances and events during their perinatal period, would help reach an early diagnosis of the possible developmental disorders they might suffer later on in life. SUBJECTS AND METHODS: Both visual and auditory cerebral evoked potentials were recorded in a group of children born underweight and the results were compared with the findings from another group of healthy children who were born in normal physiological conditions and were apparently free of any kind of pathology. RESULTS: In the waves and locations that were examined, the problem group displayed latencies that were longer than those of the control group; in contrast, no statistically significant differences were found in the amplitude, regardless of the location. Low gestational age and lower weight made latencies longer, but no relationship was found between latencies and the other perinatal features that were studied. CONCLUSIONS: Children with low weight at birth have slower wave latencies than normal children. This slowing, which is inversely proportional to the weight and weeks of gestation, is considered to be an anomalous sign that could be related to brain immaturity, delayed development or to disorders affecting myelination. Moreover, the amplitude, which has received far less attention from researchers, is usually shorter in these processes, although in our study we found no differences with the group of healthy children--only very slightly in the P300, in the weeks and the weight, and the N100 only in one location with respect to weight. Since these children usually have developmental disorders, the use of evoked potentials could be a very useful tool in their detection and ensuing therapy.