A systematic literature review was undertaken to determine if conditioned pain modulation (CPM) is reliable. Longitudinal, English language observational studies of the repeatability of a CPM test paradigm in adult humans were included. Two independent reviewers assessed the risk of bias in six domains; study participation; study attrition; prognostic factor measurement; outcome measurement; confounding and analysis using the Quality in Prognosis Studies (QUIPS) critical assessment tool [17]. Intraclass correlation coefficients (ICCs) less than 0.4 were considered to be poor; 0.4 and 0.59 to be fair; 0.6 and 0.75 good and greater than 0.75 excellent [37]. Ten studies were included in the final review. Meta-analysis was not appropriate due to differences between studies. The intersession reliability of the CPM effect was investigated in 8 studies and reported as good (ICC = 0.6-.75) in 3 studies and excellent (ICC > .75) in subgroups in 2 of those 3. The assessment of risk of bias demonstrated that reporting is not comprehensive for the description of sample demographics, recruitment strategy and study attrition. The absence of blinding, a lack of control for confounding factors and lack of standardisation in statistical analysis are common. CPM is a reliable measure, however the degree of reliability is heavily dependent upon stimulation parameters and study methodology and this warrants consideration for investigators. The validation of CPM as a robust prognostic factor in experimental and clinical pain studies may be facilitated by improvements in the reporting of CPM reliability studies.