Hospital doctors face being "bullied and harassed" if they raise concerns about patient safety, the British Medical Association has claimed.
More than 500 doctors at hospitals across Wales answered a BMA survey, with nearly 60% saying they had raised a concern in the previous three months.
Of those, more than 60% reported experiencing bullying or harassment as a result.
The Welsh government said staff concerns should be addressed.
The survey was sent to 3,000 staff including consultants, junior doctors and specialists between March and May this year, with just over one in six responding.
Dr Phil Banfield, chair of the BMA's Welsh council, called the situation "hugely worrying".
"Doctors care passionately about their patients and a key part of that is having the confidence to be able to raise concerns on their behalf," he said.
"To make this a reality we need a culture of openness within the NHS, not one where raising concerns can leave doctors feeling harassed or marginalised."
'No action taken'
Of those who raised concerns, nearly 40% reported no action being taken to the best of their knowledge.
Nearly a third said unfilled staff vacancies had caused the incident they had reported, while a quarter pointed to a higher than usual workload and just over half to systemic causes such as the drive to meet targets or inadequate facilities.
Dr Banfield added: "This survey further highlights the strain that NHS staff on the frontline are facing, with 84.8% reporting long-term unfilled staff vacancies in their workplace, and 69% agreeing that staff in their unit work longer hours than is best for patient care.
"It is imperative that these fundamental problems are addressed to create an environment where patient safety is paramount."
A Welsh government spokesman said: "We expect all NHS organisations to engage continuously with their staff and the public to ensure services are safe, sustainable and meet national clinical standards.
"All NHS Wales staff should be treated with dignity and respect, in line with established policy. All NHS organisations must take action to address any concerns raised by staff in a prompt and timely manner.
"We are pleased BMA Wales is working in partnership with other trade unions and NHS Employers on the development of core NHS values."