Aims: To assess the relation between violence prevention policies and work related assault. Methods: From Phase 1 of the Minnesota Nurses' Study, a population based survey of 6300 Minnesota nurses (response 79%), 13.2% reported experiencing work related physical assault in the past year. In Phase 2, a case-control study, 1900 nurses (response 75%) were questioned about exposures relevant to violence, including eight work related violence prevention policy items. A comprehensive causal model served as a basis for survey design, analyses, and interpretation. Sensitivity analyses were conducted for potential exposure misclassification and the presence of an unmeasured confounder. Results: Results of multiple regression analyses, controlling for appropriate factors, indicated that the odds of physical assault decreased for having a zero tolerance policy (OR = 0.5, 95% CI 0.4 to 0.8) and having policies regarding types of prohibited violent behaviours (OR = 0.5, 95% CI 0.3 to 0.9). Analyses adjusted for non-response and non-selection resulted in wider confidence intervals, but no substantial change in effect estimates.Conclusions: It appears that some work related violence policies may be protective for the population of Minnesota nurses.O ver 600 work related homicides occurred in the USA during 2002 alone, and it is estimated that approximately 1.7 million non-fatal acts of violence occurred at work annually between 1993 and 1999.1 2 Workers in certain occupations, including nursing, are at increased risk of non-fatal work related assault.3-8 Primarily, descriptive studies have been used to identify the prevalence of violence focused on particular facilities, units, or sub-groups of nurses; however, it is difficult to compare these data when definitions of violence, study populations, and study periods differed. Estimates range from 87% of staff being assaulted five times per year 9 to 57% of staff per year, 10 with numerous studies reporting percentages in between.Currently, there is no mandate, only recommendations, from the federal Occupational Safety and Health Administration (OSHA) regarding work related violence prevention measures. In 1996, OSHA published Guidelines for preventing workplace violence for health care and social service workers.11 These guidelines are advisory in nature, neither standards nor regulations. The recommendations state that employers should create ''a clear policy of zero-tolerance for workplace violence, verbal and non-verbal threats, and related actions''.11 Few data exist to support recommendations from many researchers and OSHA to implement policies;11-21 therefore, evaluation of these recommendations is necessary.It is unclear how many organisations have policies regarding work related violence.22 23 However, simply having a violence prevention policy does not ensure a safe work environment. 24 25 What is included within a violence prevention policy varies by institution. Some authors have recommended a ''zero-tolerance policy' ', 11 26-30 while others cautioned against its ...