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Tracheostomy is common in intensive care unit patients, but the appropriate timing is controversial.To determine whether earlier tracheostomy is associated with greater long-term survival.Retrospective cohort analysis.Acute care hospitals in Ontario, Canada (n = 114).All mechanically ventilated intensive care unit patients who received tracheostomy between April 1, 1992 and March 31, 2004, excluding extreme cases (<2 or ≥28 days) and children (<18 yrs).For crude analyses, tracheostomy timing was classified as early (≤10 days) vs. late (>10 days) with mortality measured at multiple follow-up intervals. Proportional hazards analyses considered tracheostomy as a time-dependent variable to adjust for measurable confounders and possible survivor treatment bias. We used stratification, propensity score, and instrumental variable analyses to adjust for patient differences.A total of 10,927 patients received tracheostomy during the study, of which one-third (n = 3758) received early and two-thirds late (n = 7169). Patients receiving early tracheostomy had lower unadjusted 90-day (34.8% vs. 36.9%; p = 0.032), 1 yr (46.5% vs. 49.8%; p = 0.001), and study mortality (63.9% vs. 67.2%; p < 0.001) than patients receiving late tracheostomy. Multivariable analyses treating tracheostomy as a time-dependent variable showed that each additional delay of 1 day was associated with increased mortality (hazard ratio 1.008, 95% confidence interval 1.004–1.012), equivalent to an increase in 90-day mortality from 36.2% to 37.6% per week of delay (relative risk increase 3.9%; number needed to treat, 71 patients to save one life per week delay).This analysis provides guidance regarding timing but not patient selection for tracheostomy.Physicians performing early tracheostomy should not anticipate a large potential survival benefit. Future research should concentrate on identifying which patients will receive the most benefit.