No medical records survive. At least three theories for the cause of death have been given by various sources: The New York Times stated he had died of heart disease « Dan Leno Dead" », . The Oxford Dictionary National Biography, on the other hand, states that he died of tertiary syphilis[1]. Finally, his biographer Gyles Brandreth claimed that Leno had succumbed to a brain tumour, which Brandreth thought would help explain his erratic behaviour. Leno stated in 1904: "the cause of my brain trouble was attributed to a fall off my bicycle"[F 20].
oxforddnb.com
Hogg, James. "Leno, Dan", Oxford Dictionary of National Biography, Oxford University Press, accessed November 2011
Flanders, Judith. "1901 census", Who Do You Think You Are magazine, accessed 27 June 2013
wikipedia.org
en.wikipedia.org
The Era, 12 février 1860. Enregistrement montrant William Leno apparaissant en Arlequin(en) dans Harlequin and the Yellow Dwarf au Theatre Royal, South Shields.
p. 45–47. Sheffield's Lyceum Theatre(en) was built on the site in 1897. See Anthony, p. 54