Some patients are going to A&E units in the UK more than 50 times a year, a BBC investigation shows.
Data from 183 sites obtained under the Freedom of Information Act revealed nearly 12,000 people made more than 10 visits to the same unit in 2012-13.
A small number of those - just over 150 - attended more than 50 times.
Doctors warned that the issue, while confined to a minority of patients, was adding to the mounting pressures on the system this winter.
Many hospitals are already struggling to hit their waiting-time targets.
In England, the four-hour target was missed in the first two weeks of December, but the numbers attending A&Es eased over the Christmas period and the target has been met for the past fortnight.
However, that overall figure masks the struggle the biggest units are facing.
Of the 144 major centres, more than 60 failed to hit the target during the Christmas week despite the number of patients dropping by nearly 10%.
Dr Cliff Mann, of the College of Emergency Medicine, said the BBC's findings on repeat attendees raised some worrying questions as hospitals struggled to cope.
He said there were a variety of issues which led patients to become frequent visitors, including mental health and problems such as drug and alcohol abuse, which suggested that with better support in the community, particularly from social care, repeat visits could be prevented.
But he also said others were using A&E simply because they had had good experiences in the past, while those who struggled with the language may be using A&E units as a default because they found it hard to navigate their way round the system.
"At a time when emergency departments are very busy, it is clear that repeat non-emergency attendees are adding to the pressures," he added.
As part of the investigation, the BBC asked 175 hospital trusts and boards across the UK for information on repeat attendances during 2012-13. A total of 106 responded, covering 183 units.
Among the worst cases identified was a patient at Luton and Dunstable Hospital who attended 234 times in the year, while one person went to Sheffield's Northern General 223 times.
Patients Association chief executive Katherine Murphy said: "These figures are of great concern and provide yet more proof that our already overstretched A&E services are being put under increasingly severe strain.
"It needs to be recognised that every inappropriate attendance at the A&E would detract health professionals from attending to the genuine and often life-threatening emergencies," she added.
She said improving access to GPs could make a big difference - before Christmas, official NHS data showed the number of people waiting longer than a week to be seen by a GP was growing.
Andy Bell, of the Centre for Mental Health, said: "These figures show how vital it is for every hospital with an A&E unit to have 24/7 access to mental health advice and support. Too many people with mental health problems struggle to get the help they need in a crisis."
A spokeswoman for NHS England said the figures should be seen in context - in England alone there were more than 21 million visits to A&E units last year.
"The figures presented here suggest that the number of people attending A&E frequently is relatively small when considered against the big picture."
Meanwhile, a spokeswoman for the Department of Health said steps were being made to improve access to community care through the launch of a £3.8bn joint pot in April 2015 to encourage the NHS and local government to work together.
A series of 14 pilots are already under way to pioneer new projects.
"We know action is needed to tackle pressures on A&E," she added.