OBJECTIVES: Assess the frequency and reasons for disagreements in risk of bias assessments for randomised controlled trials (RCTs) included in more than one Cochrane review. DESIGN: Research on research study, using cross-sectional design. DATA SOURCES: 2796 Cochrane reviews published between March 2011 and September 2014. DATA SELECTION: RCTs included in more than one review. DATA EXTRACTION: Risk of bias assessment and support for judgement for five key risk of bias items. DATA SYNTHESIS: For each item, we compared risk of bias assessment made in each review and calculated proportion of agreement. Two reviewers independently analysed 50% of all disagreements by comparing support for each judgement with information from study report to evaluate whether disagreements were related to a difference in information (eg, contact the study author) or a difference in interpretation (same support for judgement but different interpretation). They also identified main reasons for different interpretation. RESULTS: 1604 RCTs were included in more than one review. Proportion of agreement ranged from 57% (770/1348 trials) for incomplete outcome data to 81% for random sequence generation (1193/1466). Most common source of disagreement was difference in interpretation of the same information, ranging from 65% (88/136) for random sequence generation to 90% (56/62) for blinding of participants and personnel. Access to different information explained 32/136 (24%) disagreements for random sequence generation and 38/205 (19%) for allocation concealment. Disagreements related to difference in interpretation were frequently related to incomplete or unclear reporting in the study report (83% of disagreements related to different interpretation for random sequence generation). CONCLUSIONS: Risk of bias judgements of RCTs included in more than one Cochrane review differed substantially. Most disagreements were related to a difference in interpretation of an incomplete or unclear description in the study report. A clearer guidance on common causes of incomplete information may improve agreement.