ABSTRACT OBJECTIVEâ€”We assessed the effect upon A1C of recruitment to a clinical trial in patients with diabetes who had been screened and interviewed to determine eligibility but whose therapy was otherwise unchanged. RESEARCH DESIGN AND METHODSâ€”Eligible trials were selected from the global program of an insulin manufacturer. Included were studies in which patients were seen on a single screening visit, pharmaceutical therapy was not altered before randomization, and A1C was measured in a central laboratory at both screening and randomization. Three trials involving patients with type 1 diabetes (n = 429) and three trials involving patients with type 2 diabetes (n = 611) were identified for analysis. The main outcome measure was change in A1C. Separate regression equations on the change in A1C were fitted for type 1 and type 2 diabetes and included effects of baseline A1C and the interval between the screening and randomization visits. RESULTSâ€”A1C changed by â€“0.13% (range +0.09 to â€“0.26%) in those with type 1 diabetes at a median of 28 days and by â€“0.16% (â€“0.14 to â€“0.27%) for those with type 2 diabetes at a median of 14 days. The mean change in A1C in those with an interval of â‰¥28 days was â€“0.24% for those with type 1 diabetes and â€“0.23% for those with type 2 diabetes. The reduction was proportional to initial A1C, with large decreases in those with the poorest initial control but no overall change in those at or below the 10th percentile of A1C. CONCLUSIONSâ€”Recruitment to a clinical trial, independent of any therapeutic intervention, produces improvements in glucose control.