The frequency of violent behaviour in mental hospitals has been increasing in recent years. A number of factors may be responsible. Violent and dangerous patients are sent to hospitals, quite often against their will. This may lead to conflicts and assaults against the staff members. There are many factors, both in present situation and in biography, conductive to violent behaviour: unfavourable experiences in childhood (neglect, cruelty, sexual exploitation), psychopathic structure of premorbid personality, frustrations, and eventually deformations of world perception caused by psychotic symptoms. Various mental disorders may lead to the violent behaviour, but it is most frequently observed in exacerbation of paranoid schizophrenia, in young males, particularly in cases with systemized delusions, emotional turmoil and anger. Introduction of a person (nurse, physician, family member, other patient) into psychotic world may also lead to the attack. In particular cases it is difficult to foresee violent behaviour, but some indicators are known. There are very few investigations on the role of the staff in violent behaviour of patients. The danger may be brought by criticism, refusal and rejection, compulsory drug administration, undue limitations of the patient's liberty, or the opposite--no reaction to violations of institutional regulations.