More needs to be done to improve standards of safety in the NHS and independent sector, a watchdog says.
The Healthcare Commission said that while most patients received safe care, standards were inconsistent in England and Wales.
The watchdog said there was no clear indication on the number of deaths that could be avoided.
It also raised concerns about a range of other areas, including mental health care and health inequalities.
The annual State of Healthcare report draws on data from surveys, focus groups and inspections.
The report said patient satisfaction levels remained high, and waiting lists had fallen in the last five years.
But it said attention had to be focussed on four areas: safety; designing services around patients; reducing inequalities in health; and tackling services for people with mental health problems, learning disabilities, older people and children.
Healthcare Commission Chairman Sir Ian Kennedy said: "There are real improvements to applaud and celebrate. These are having a genuine impact on people's lives with many waiting less time for care and experiencing better treatment."
But he added: "It's frustrating that in 2006 we do not have a clearer idea of how many people die or are harmed when this could have been avoided."
To illustrate his point, he said the National Audit Office could only estimate the number of deaths as a result of patient safety incidents ranged from 840 to 34,000.
The report revealed in the NHS, one in 10 trusts could not confirm that they fully met core standards on safety.
In the independent sector, one in 10 providers were ordered to improve the management of risks last year.
And the Healthcare Commission said 22% of the complaints it handles related to safety.
Safety includes areas such as infection control and drug administration.
The report also marks the first publication of an overview of standards in the independent sector in England, which includes both private and voluntary providers.
Some 50% of providers met all 32 minimum standards, but one in 10 failed five or more - broadly in-line with NHS organisations.
Jo Webber, deputy director of policy at the NHS Confederation, said some things were improving with many trusts now actively encouraging their staff to report incidents.
"Communication to staff and patients is key here, but can often be a challenge in large and complex organisations."
And shadow health secretary Andrew Lansley added: "It is a shame that this willingness to improve patient safety is not shared by the Department of Health.
"In December 2003, the Chief Medical Officer ordered a national audit of deaths caused by hospital-acquired infections. We are still waiting for it to be published."