The major finding of the present meta-analysis is that the presence of subjectively reported post-exertional malaise is 10.4 times more likely to be associated with an ME and CFS diagnosis than with control status. This finding can reasonably be considered robust to publication bias, and strongly suggests that self-reported PEM discriminates well between ME and CFS and controls and has meta-analytic support as a cardinal symptom of the disease. The hypothesis that PEM and ME and CFS would be significantly associated is supported. Thus, case definitions that require PEM for a diagnosis may be most appropriate for use (e.g., Carruthers et al., 2003) and should be relied upon rather than the most commonly utilized polythetic Fukuda et al. (1994) criteria.