Health campaigners have welcomed a report which shows some NHS trusts have a fragmented and inconsistent approach to patient complaints.
The Healthcare Commission issued warnings to 30 trusts, and told 12 of them there had been a "significant lapse" in how they handled problems.
An investigation found some complaints were ignored, while other people waited months for their issue to be assessed.
The NHS provides around 380 million treatments each year, and receives around 140,000 formal complaints.
But the charity Action against Medical Accidents said more needs to be done to address patients' concerns.
Chief executive, Peter Walsh, said: "This audit is further evidence, as if we needed it, that the way many NHS organisations handle complaints adds insult to injury and there is an urgent need for improvement.
"The government's proposals for reform offer some hope of a fresh new response which must be seized.
"However, we are concerned that the loss of important safeguards, such as having the right to an independent review of complaints by the Healthcare Commission, could leave many complainants with nowhere to go."
A spokeswoman for the Department of Health said the NHS receives a "tiny number" of complaints in relation to the number of treatments.
"But when patients do complain, their complaint should be handled quickly and effectively," she said.
"This is something we expect every NHS organisation to take seriously."