Post-traumatic stress disorder (PTSD) has been called one of the signature injuries of the Iraq War. In this review prevalence estimates of PTSD are summarized and discrepancies are discussed in relation to methodological differences between studies. We searched for population-based studies with a minimum sample size of 300. Studies based on help-seeking samples were excluded. We identified 60 possible papers, of which 19 fulfilled the inclusion criteria. Prevalence estimates and study characteristics were examined graphically with forest plots, but because of high levels of heterogeneity between studies, overall estimates of PTSD prevalence were not discussed. The prevalence of PTSD in personnel deployed to Iraq varied between 1.4% and 31%. Stratifying studies by PTSD measure only slightly reduced the variability in prevalence. Anonymous surveys of line infantry units reported higher levels of PTSD compared to studies that are representative of the entire deployed population. UK studies tend to report lower prevalence of PTSD compared with many US studies; however, when comparisons are restricted to studies with random samples, prevalences are similar. US studies that have assessed personnel more than once since return from deployment have shown that PTSD prevalence increases over the 12 months following deployment. Differences in methodologies and samples used should be considered when making comparisons of PTSD prevalence between studies. Further studies based on longitudinal samples are needed to understand how the prevalence of PTSD changes over time.