Nine patients with lung cancer without brain metastasis were studied pre and post brain prophylactic radiotherapy (30 Gy) to verify the effect of radiation on the cognitive processes. The P3 component of event-related potentials was used in this study. The latencies pre and post-radiation were compared and a significant delay (p less than 0.01) was detected in patients after treatment. These results suggest that brain radiotherapy may produce a cognitive impairment, as reported by many Authors.