Readers of randomized controlled trials (RCTs) commonly assume that what was not reported did not occur. We undertook an observational study to determine whether concealment of randomization or blinding was used in RCTs that failed to report these bias-reducing strategies. We recorded the reporting of concealment of randomization and blinding in 105 RCTs. We subsequently contacted the authors and determined if they had used these methodological safeguards. We successfully obtained data from 98 authors. The authors in the full-text publications of these 98 RCTs failed to report the presence or absence of concealment of randomization in 55%, and the blinding status of participants in 26%, health care providers in 64%, data collectors in 84%, outcome assessors in 83%, and data analysts in 96%. In direct contact, authors frequently reported concealing randomization (96%; 95% confidence interval CI = 87-100%), blinding participants (20%; 95% CI = 7-41%), blinding health care providers (65%; 95% CI = 52-77%), blinding data collectors (65%; 95% CI = 53-75%), blinding outcome assessors (79%; 95% CI = 69-87%), and blinding data analysts (50%; 95% CI = 40-60%), despite not reporting the use of these methodological safeguards in their publications. Readers should not assume that bias-reducing procedures not reported in an RCT did not occur. © 2004 Elsevier Inc. All rights reserved.