Telling your boss what you really think of them is good for your health and helps managers improve, according to research published on Wednesday.
Firms should be even be encouraged to let employees regularly rate their line managers, to produce "happy, healthy, stress-free employees," said the study presented at a conference of the British Psychological Society.
Researchers split a group of 150 bosses into two groups, one of which received feedback on their management skills from some 500 staff, and another group which did not.
"When managers received feedback from their staff, they were more likely to change their management style and subsequently be seen as more effective line managers," said the study.
Employees benefit as it allows them to let off steam, said expert Emma Donaldson-Feilder, presenting the research at a conference in Brighton.
"The consequences of stress are pervasive; those under stress may experience psychological symptoms, such as anxiety or depression, physiological symptoms, such as palpitations or raised blood pressure and/or cognitive symptoms such as reduced mental capacity.
"Stress is a significant cause of sickness absence and this puts pressure on those left behind to run the business, creating a cycle of uncomfortable pressure with costs to the individual and to the company," she said.
Donaldson-Feilder and her colleagues are developing a number of resources including a questionnaire that staff can use to rate their line manager and learning materials for managers, which will be available free online.
"Without holding a mirror up to a person, they can have blind spots about how they come across and if they think they are already good enough, why should they change," she said.