Background: Blinding of persons involved in randomised clinical trials minimises bias. Unfortunately, the description of blinding in trial reports is often inadequate for identification of who is intended to be blind and by which mechanisms. Protocols to trials provide a possible source of reliable information on blinding. Objectives: To compare the description of blinding in corresponding pairs of protocols and trial reports to randomised controlled trials. Methods: We studied 73 protocols of randomised and blinded trials approved by the scientific and ethical committees for Copenhagen and Frederiksberg, 1994 and 1995, and their corresponding publications. Results: Three out of 73 trials (4%) reported blinding in the protocol that contradicted that in the publication (e.g. ‘open’ vs. ‘double blind’). There were 58 trials labelled ‘double blind’. The proportion of such trials with a clear description of the blinding of participants increased from 11 (19%) when based on publications alone to 39 (67%) when adding the information in the protocol. The similar proportions for the blinding of healthcare providers were 2 (3%) and 22 (38%), and for the blinding of data collectors 8 (14%) and 14 (24%). In 52 out of 58 publications (90%), it was unclear whether all patients, healthcare providers, and data collectors had been blinded. In 4 of the 52 trials (7%), the protocols clarified that all three key trial persons had been blinded. Conclusions: The reporting on blinding in trial publications is often inadequate. The reporting on blinding in protocols is more comprehensive, but with considerable room for improvement. We suggest developing international guidelines for the reporting of trial protocols and public access to protocols.