Woolfe, a Member of the European Parliament (MEP), suffered two seizures hours after what the party described as an "altercation" with fellow UKIP members in the European Parliament.
According to reports, Woolfe was punched by his party colleague Mike Hookem.
"I deeply regret that following an altercation that took place at a meeting of UKIP MEPs this morning that Steven Woolfe subsequently collapsed and was taken to hospital. His condition is serious," UKIP interim leader Nigel Farage, who spearheaded the Leave campaign during the Brexit referendum, said in a statement.
Woolfe issued a statement from hospital in Strasbourg: "The CT scan has shown that there is no blood clot in the brain. I am feeling brighter, happier and smiling as ever. I am sitting up and said to look well.
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"The only consequence at the moment is a bit of numbness on the left hand side of my face. I am being kept in overnight awaiting secondary tests to make sure everything is fine."
UKIP MEP Roger Helmer told the BBC the incident followed "some lively exchanges" at a closed meeting of party MEPs today morning.
He said the gathering happened "a good two hours" before a voting session from which he understood Woolfe left.
The latest incident comes against a background of bitter rivalry and feuding in the UKIP ranks, with many senior figures openly at odds with each other.
Feelings were already running high, with some in the anti-immigrant party deeply unhappy at Farage's decision to carry on as interim leader until a permanent successor was in place.