This paper examines the impact of prison conditions on future criminal behaviour. The analysis is based on a unique dataset on the post-release behaviour of 25,000 Italian former prison inmates. We use an exogenous variation in prison assignment as a means of identifying the effects of prison overcrowding, deaths in prison, and degree of isolation on the probability of re-offending. We do not find compelling evidence of (specific) deterrent effects of prison severity. The measures of prison severity do not affect negatively the probability of recidivism. Instead, all point stimates suggest that harsh prison conditions increase postrelease criminal activity, though they are not always precisely estimated.