Mick Bates at the Welsh Liberal Democrat conference in Swansea
Liberal Democrats are investigating allegations about the behaviour of a senior member of the Welsh assembly in hospital after a night out in Cardiff.
Welsh party leader Kirsty Williams is trying to establish the facts after a newspaper claim that Montgomeryshire AM Mick Bates verbally abused NHS staff.
She said she understood he was knocked unconscious and had a serious head injury which still caused him problems.
Mr Bates said he could not remember what had happened.
Wales on Sunday said the 62-year-old AM was admitted to the University Hospital of Wales, Heath, early on 20 January.
A spokesman for Cardiff and Vale University Health Board said he could not confirm whether an investigation was launched.
"We would not comment on an individual patient," he said.
"But if there were complaints or allegations they would be fully investigated."
The claims emerged as the party held its Welsh spring conference in Swansea, and it said it took the allegations "very seriously".
Mr Bates was due to speak at a fringe meeting at the conference, but he was replaced by Lembit Opik, the MP for his constituency.
Ms Williams told the BBC Wales Politics Show she took the allegations "very seriously," and she would contact the NHS trust involved.
"We need to establish the facts," she said.
"I've spoken to Mick Bates - he's apologised if his behaviour following the incident has caused any concerns."
When asked about whether he still had her confidence, she replied: "Yes he does.
"We don't know the facts - we need to get to the bottom of this. My understanding is that Mick was knocked unconscious and suffered a severe head injury - which he's continuing to have problems with."
She added: "Abuse towards NHS staff is not acceptable so we need to talk to the trust to establish the facts".
Mr Bates told BBC Wales that he could not remember what had happened on the night.
Earlier, a spokesperson for the Welsh Liberal Democrats said: "We take these allegations very seriously and we would not condone this sort of alleged behaviour.
"We have already begun an investigation to establish the facts and we will return with a further statement once our investigations are concluded."
Mr Bates, who has represented Montgomeryshire since the assembly was founded in 1999, has been chairman of its sustainability committee since 2007.
A former teacher who became a farmer, he said last year that his wife, a headmistress, was retiring in 2011, and that it was an "ideal time" for him to stand down.
Mr Bates is also a former county chairman of the National Farmers' Union, and became a local councillor in 1994.
He held his assembly seat at the 2007 election with a majority of just over 2,000 ahead of the Conservatives.