Self-reported pain intensity, frequently used as an outcome in randomized clinical trials (RCTs) of chronic pain treatments, is often highly variable and could be associated with multiple baseline factors. Thus, the assay sensitivity of pain trials (i.e., the ability of the trial to detect a true treatment effect) could be improved by including such pre-specified baseline factors in the primary statistical model. The objective of this systematic review is to characterize the baseline factors that are used for adjustment in statistical analyses of chronic pain RCTs published in 6 major pain journals.