Napoleon's illness and death on St Helena have been the subject of controversy. His symptoms have been compared with those of arsenic poisoning1-3, and arsenic has been found in surviving samples of his hair4-8. During the nineteenth century, many people were accidentally poisoned by arsenical vapours from wallpaper9-11. Accordingly, it is an intriguing speculation that Napoleon may have ingested arsenic from his wallpaper. After presenting this speculation on a radio broadcast, one of us (D.E.H.J.) was offered an original sample of wallpaper from Napoleon's residence on St Helena. We report here that X-ray fluorescence measurements on this paper reveal enough arsenic to be capable of causing illness but probably not death. So if Napoleon was poisoned by arsenic, it may have come not from deliberate administration but from his wallpaper.