Beethoven was a victim of lead poisoning, according to an analysis of samples of his hair carried out by researchers in Chicago.
The verdict, after four years of research, could provide a clue as to why the composer sometimes behaved in strange and eccentric ways and may also be an explanation for his deafness.
The findings, made by scientists attached to the Health Research Institute in Argonne, Chicago, clear up speculation surrounding the possible source of his illness.
Analysis of his hair clippings showed that he had around 100 times the level of lead that would be normal in a person today.
It is unclear how he came to be so badly poisoned, although one theory is that he could have ingested the lead through mineral waters taken at the many spas he visited.
"It was a surprise, but it stood out like a sore thumb in the analysis," said Mr William Walsh, the director of the research project.
The analysis was carried out on hairs that were supposedly taken from Beethoven after his death at the age of 56, in 1827. The hairs were purchased for $7,300 in 1994, at Sotheby's, by Ira Brilliant of the Centre for Beethoven Studies at San Jose State University.
The 582 strands were said to be a mixture of grey and brown. Hairs were taken from Beethoven's body by many people prior to his burial.
"He was shorn. He was practically bald when he was buried," said Mr Brilliant, founder of the centre, who bought the hair with Alfredo Guevara, a surgeon from Nogales, Arizona.
The findings also clear up speculation as to whether Beethoven suffered from syphilis. There was no trace of mercury, then the common method of treating the disease. Nor were there any traces of the painkillers used at the time.
Beethoven had constantly complained of illness during his life. But whether the lead poisoning caused his deafness is not certain. "That's really the million-dollar question," Mr William Meredith, the director of the centre, said.