Clinical evidence shows that pain, particularly without organic lesions, tends to be more frequent on the left side of the body. Concomitantly, experimental data indicate that differential sensitivity for the right and left side, the pain threshold and tolerance being usually higher in the dominant than in the non-dominant side of the body. Headache laterality was investigated in 188 consecutive chronic headache patients. Headache was lateralized in approximately 50% of the cases, with an overall right predominance (59%). However, when descriptive diagnosis was taken into consideration, lateralized pain was predominant on the left side in psychogenic headache (p less than .01), whereas somatogenic headache was more often right-sited. Since pain lateralization can hardly be explained by peripheral factors, we assumed that reduced efficiency of the non-dominant side in processing several perceptual and cognitive activities could reflect asymmetry in cerebral organization. As differential engagement of the two hemispheres by appropriate tasks can be demonstrated with the EEG, we have studied 13 chronic headache patients with quantitative EEG analysis, compared with a control group of normal subjects (n = 7). The analysis of alpha-asymmetry, during analytical and spatial tasks, revealed that chronic headache patients showed a trend toward a relative activation of the right hemisphere during both tasks, independently of descriptive diagnosis, suggesting that chronic headache might be considered a "psychosomatic continuum". We postulate that pain perception may be biased by the reduced efficiency of the right hemisphere in sensory processing, so that the sensory message may be misinterpreted or amplified as painful by higher centers, even in absence of nociceptive stimuli.