Purpose: To assess reporting completeness of the most frequent outcome measures used in randomized controlled trials (RCTs) of rehabilitation interventions for mechanical low back pain. Methods: We performed a cross-sectional study of RCTs included in all Cochrane systematic reviews (SRs) published up to May 2013. Two authors independently evaluated the type and frequency of each outcome measure reported, the methods used to measure outcomes, the completeness of outcome reporting using a eight-item checklist, and the proportion of outcomes fully replicable by an independent assessor. Results: Our literature search identified 11 SRs, including 185 RCTs. Thirty-six different outcomes were investigated across all RCTs. The 2 most commonly reported outcomes were pain (n=165 RCTs; 89.2%) and disability (n=118 RCTs; 63.8%), which were assessed by 66 and 44 measurement tools, respectively. Pain and disability outcomes were found replicable in only 10.3% (n=17) and 10.2% (n=12) of the RCTs, respectively. Only 40 RCTs (21.6%) distinguished between primary and secondary outcomes. Conclusions: A large number of outcome measures and a myriad of measurement instruments were used across all RCTs. The reporting was largely incomplete, suggesting an opportunity for a standardized approach to reporting in rehabilitation science.