Argonne National Laboratory announced href="http://www.anl.gov/Media_Center/News/2005/news051206.html">finding that Beethoven's illness was caused by lead poisoning.
According to the press release, bone fragments from Beethoven's body (confirmed by DNA testing) were shown to contain large amounts of lead, relative to another bone fragment from another person of the same period. The bone fragments come from the top of the skull. The analysis is performed via elemental X-ray fluorescence at the Advanced Photon Source.
The presence of lead in the skull meant that he might have suffered from the condition for much of his life. Together with described physiological symptoms, it is almost certain that Beethoven suffered from lead poisoning.