A Leeds United fan was taken to hospital in London on Wednesday night after falling from the upper tier at Chelsea's Stamford Bridge.
Medical personnel rushed to the scene and provided first aid before the fan was escorted away.
Referee David Coote did not stop the game.
The incident occurred moments after Mateo Joseph had fired Leeds into a 1-0 lead eight minutes into the game in the fifth round of the FA Cup.
A 51-word club statement issued during the second half read: "Leeds United can confirm an incident involving a Leeds United supporter occurred in the Shed End of Stamford Bridge during the first-half of tonight's FA Cup game.
"The incident is being investigated and is in the hands of the emergency services.
"We will be making no further comment at this time."
Leeds had sold their full allocation of 5,366 tickets for Wednesday's game, with away fans housed in both the upper and lower tiers of the Shed End.
Chelsea won the match 3-2 to progress to the quarter-finals of the FA Cup, where they will host Leicester City.
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