A health watchdog has accused the NHS of letting down patients who complain about their care.
The Parliamentary and Health Service Ombudsman's investigation looked into 150 complaints regarding allegations that patients had died or suffered avoidable harm because of failings in their care.
It found that 28 of the 150 cases should have been investigated by the NHS as a Serious Untoward Incident (SUI), which is triggered to allow doctors to learn from past mistakes.
But in the vast majority of these cases (71%) the hospital failed to carry out an SUI.
Families interviewed for the review complained they felt "belittled" and "misled" by medical staff who failed to listen to their concerns or give them straight answers.
Patients groups said the findings are "worrying" and suggest the lessons from recent hospital scandals have not been learnt.
Ombudsman Dame Julie Mellor said: "Investigations weren't carried out when they should have been and when they were carried out they did not find out or explain why failings happened.
"When people make a complaint that they have been seriously harmed they should expect it to be taken seriously and thoroughly investigated."
In one case highlighted, a one-day-old baby, identified only as Baby F, was left with permanent brain damage because two doctors and a nurse made "serious mistakes" during blood transfusions.
But the hospital's SUI investigation was "fundamentally flawed and did not identify glaring errors in the conduct and recording of the transfusion and ignored obvious explanations for what happened", the ombudsman found.
Dr Katherine Rake, chief executive of Healthwatch England, said: "Our research shows that tens of thousands of people every year are being failed by the NHS and yet never report it because they have no faith the complaints system will make any difference.
"Even worse, fewer than half of those that do find the energy and courage to raise their concerns ever hear the words 'I'm sorry'.
"What we need is a complete overhaul of the complaints system that ensures every incident is properly investigated and learnt from, and that those affected are treated with the dignity they deserve."
A Department of Health spokesman said the NHS acknowledges that "listening to patients and staff is absolutely vital to improving care".
He added: "That's why we've made hospitals legally obliged to apologise to patients when mistakes do happen, introduced complaints handling as a crucial element of tougher hospital inspections and asked Robert Francis to produce an independent report on how to create a more open NHS culture."