Background: The methods to assess the success of blinding in randomized controlled trials (RCTs) has been widely debated. However, no rigorous study of assessing the success of blinding has been published.

Objective: To determine methods to assess the success of blinding in randomised controlled trials (RCTs).

Methods: We searched MEDLINE, the Cochrane Controlled Trials Register and the Cochrane Method Register and performed a manual search to target studies that attempt to assess blinding and describe the methods used in those studies.

Results: A total of 90 reports were selected. Reports assessed the success of blinding participants (n = 58), care providers (n = 36) and outcome assessors (n = 15). Of the 58 reports assessing the success of blinding participants, 54 (93%) reported asking participants to guess their treatment assignment. The timing of assessment (e.g., once at the end of the trial [57%] or several times during the trial [26%]) and modalities of answering (e.g., do not know answers [43%] or participants forced to guess [31%]) were inconsistent. A statistical analysis was performed in 57% of reports. The statistical analysis mainly compared the proportion of correct guesses to those produced by chance (32%) or checked for a relation between participants' guesses and treatment assignment (23%).

Conclusions: Methods of assessing the success of blinding, analysis and reporting the results were inconsistent and questionable.