Supermax prisons have been advanced as means of controlling the “worst of the worst” and making prisons safer places to live and work. This research examined the effect of supermaxes on aggregate levels of violence in three prison systems using a multiple interrupted time series design. No support was found for the hypothesis that supermaxes reduce levels of inmate‐on‐inmate violence. Mixed support was found for the hypothesis that supermax increases staff safety: the implementation of a supermax had no effect on levels of inmate‐on‐staff assaults in Minnesota, temporarily increased staff injuries in Arizona, and reduced assaults against staff in Illinois.

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This research explores the sociopolitical context of prison violence and its control in the state of Illinois, and discusses the series of events that led to the opening of a supermax prison. Interrupted time series analyses were used to test whether the use of the supermax was associated with declines in prison violence, controlling for the potentially confounding influence of a systemwide effort to restructure the Illinois Department of Corrections following a prison scandal in 1996. There was no association between the opening of a supermax and inmate-on-inmate assaults; however, the supermax appears to have resulted in an abrupt, permanent reduction in assaults against staff. The opening of the supermax was also associated with an abrupt, permanent reduction in the use of lockdown days.