The man was found to have fallen from his bicycle in Askham Lane at around 1pm on Tuesday, June 8.
Emergency services attended the scene and the man was taken to Leeds General Infirmary by air ambulance.
North Yorkshire Police say the incident happened on Askham Lane, between the junctions of Cornlands Road and Tedder Road, which was closed while officers investigated.
A force spokesman said: "The cyclist was found to have fallen from his bicycle.
"There do not appear to have been any other vehicles involved, although this cannot be entirely ruled out at this stage."
Anyone with information is urged to call police on 101 quoting reference number 12210135005.