Danger of recruitment bias David J Torgerson, York Trials Unit 24 January 2008 This cluster randomised trial is at risk of incurring recruitment bias. Because primary care staff are trained in the intervention before randomisation then those in the intervention group may, due to the training, recruit different kinds of patients compared with the control group, thus introducing selection bias. This is a widespread problem in cluster randomised trials (See Puffer S, Torgerson DJ, Watson J. (2003) Evidence for risk of bias in cluster randomised trials: a review of recent trials published in three general medical journals. British Medical Journal 327, 785 and Farrin A, Russell I, Torgerson DJ, Underwood M. (2005) Differential recruitment in a cluster randomized trial in primary care: the experience of the UK Back pain, Exercise, Active management and Manipulation (UK BEAM) feasibility study. Clinical Trials 2, 119-124). The authors should, ideally, identify the patients before randomising the practices to maintain allocation concealment or use independent people in both practices (blind to the allocation) to do the recruiting. Unless this is done, then we cannot be sure that the participants are similar in unknown or unmeasured covariates. Competing interests None.