US Republican presidential hopeful Rudy Giuliani has criticised the record of the NHS during a visit to London.
Mr Giuliani said Mr Brown was very popular in the US.
The ex-mayor of New York, who suffered prostate cancer, said he would have been less likely to have survived had he been treated in the UK.
After spending the morning with Gordon Brown in Downing Street, he said the prime minister was "very popular" in the US - just as Tony Blair was.
Mr Giuliani was New York's mayor during the attacks on 11 September 2001.
Speaking at an event at a London hotel, he said: "Healthcare right now in America - and I think it has been true of your experience of socialised medicine in England - is not only very expensive, it's increasingly less effective.
"I had prostate cancer seven years . My chance of survival in the US is 82%; my chance of survival if I was here in England is below 50%. Breast cancer is very similar.
"I think there's something to the idea that there are many more private options driving the system that create altogether better results."
Mr Giuliani said he had discussed the question of Iran's nuclear ambitions "at some length and in quite a realistic way" with Mr Brown at Downing Street and had been "very, very much heartened by how seriously he sees it".
"We will use any options we believe is in our best interest to stop them becoming a nuclear power," he said.
He called on Britain to help apply "maximum pressure" to ensure Iran complied, adding: "With the absolute assurance that if they get to the point that they are going to become a nuclear power, we will prevent that or set them back five or 10 years.
"That is not said as a threat. That should be said as a promise," he said.
Mr Giuliani, who was also set to meet Tory leader David Cameron, said the so-called special relationship between Britain and the US would remain in place if he was to become the next president.
"It is now such a strong relationship that it is going to endure whatever we have to endure to overcome Islamic terrorism," he said.
Ex-British prime minister Lady Thatcher is set to listen to Mr Giuliani later when he gives an inaugural lecture in her name, organised by the Atlantic Bridge Organisation, which was set up to strengthen policy ties between conservative politicians across the Atlantic.