Context When clinicians assess the validity of randomized controlled trials (RCTs), they commonly evaluate the blinding status of individuals in the RCT. The terminology authors often use to convey blinding status (single, double, and triple blinding) may be open to various interpretations. Objective To determine physician interpretations and textbook definitions of RCT blinding terms. Design and Setting Observational study undertaken at 3 Canadian university tertiary care centers between February and May 1999. Participants Ninety-one internal medicine physicians who responded to a survey. Main Outcome Measures Respondents identified which of the following groups they thought were blinded in single-, double-, and triple-blinded RCTs: participants, health care providers, data collectors, judicial assessors of outcomes, data analysts, and personnel who write the article. Definitions from 25 systematically identified textbooks published since 1990 providing definitions for single, double, or triple blinding. Results Physician respondents identified 10, 17, and 15 unique interpretations of single, double, and triple blinding, respectively, and textbooks provided 5, 9, and 7 different definitions of each. The frequencies of the most common physician interpretation and textbook definition were 75% (95% confidence interval [Cl], 65%-83%) and 74% (95% Cl, 52%-90%) for single blinding, 38% (95% Cl, 28%-49%) and 43% (95% Cl, 24%-63%) for double blinding, and 18% (95% Cl, 10%-28%) and 14% (95% Cl, 0%-58%) for triple blinding, respectively. Conclusions Our study suggests that both physicians and textbooks vary greatly in their interpretations and definitions of single, double, and triple blinding. Explicit statements about the blinding status of specific groups involved in RCTs should replace the current ambiguous terminology.