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To highlight the limitations of single-center trials in critical care, using prominent examples from the recent literature; to explore possible reasons for discrepancies between these studies and subsequent multicenter effectiveness trials; and to suggest how the evidence from single-center trials might be used more appropriately in clinical practice.Topical and illustrative examples of the concepts discussed including trials of patient positioning, the use of steroids for acute respiratory distress syndrome, the dose of hemofiltration, the control of glycemia, and the targets of resuscitation in sepsis.Many positive single-center trials have been contradicted when tested in other settings and, in one case, the subsequent definitive multicentered trial has found a previously recommended intervention associated with active harm. Problems inherent in the nature of single-center studies make recommendations based on their results ill advised. Single-center studies frequently either lack the scientific rigor or external validity required to support widespread changes in practice, and their premature incorporation into guidelines may make the conduct of definitive studies more difficult.We recommend that practice guidelines should rarely, if ever, be based on evidence from single-center trials. Physicians should apply the findings of single-center trials only after careful evaluation of their methodology, and in particular after comparing the context of the trial with their own situation.