Pipe extruded from polyethylene of strength class PE100 was tested using the ISO 13477 Small Scale Steady State S4 method, to investigate the effect of frozen-in stress on rapid crack propagation (RCP). It was found that the lower the residual stress, the lower the S4 critical temperature TcS4 for RCP. Different experimental thermal treatments were used to independently modify residual stresses and crystallinity, to study the separate effects on RCP. It was found that the effect of crystallinity was less significant than that of residual stresses. It has previously been suggested that the residual stress influence on TcS4 is determined by the balance of two mechanisms: additional stored strain energy prior to fracture helps to drive the crack, while the closing moment after fracture helps to close the flaring pipe wall. Tests on annealed specimens suggested that the first effect dominates, especially soon after crack initiation. However, the observed effect seems too large to be explained by this mechanism, and we suggest that the observed benefit from annealing may be better explained by a change in crack front shape. POLYM. ENG. SCI., 2013. (c) 2012 Society of Plastics Engineers