Suicide rates in prisons are high. Our aim was to investigate the contribution of imitative suicide to the prison suicide rate. We used Knox tests for space-time clustering in a case register of natural and self-inflicted deaths in prisons in England and Wales and model simulations to estimate the effect size. We found significant space-time clustering among 657 self-inflicted deaths in 90 prisons over 10 years but no space-time clustering among 430 deaths from natural causes in 87 prisons over this period. Model simulations with an imitation rate of 5.8% (CI 1%-11%) reproduced the observed space-time clustering.