BACKGROUND: Blinding patients in clinical trials is a key methodological procedure, but the expected degree of bias due to nonblinded patients on estimated treatment effects is unknown. METHODS: Systematic review of randomized clinical trials with one sub-study (i.e. experimental vs control) involving blinded patients and another, otherwise identical, sub-study involving nonblinded patients. Within each trial, we compared the difference in effect sizes (i.e. standardized mean differences) between the sub-studies.