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Relationships between day of the week of admission to hospitals and hospital outcomes have been poorly studied. Intensive care units (ICUs) appear to be uniquely suited to examine such a question given the unpredictability of ICU admissions and the clinical instability of their patient populations.This retrospective cohort study included 156,136 patients admitted to 38 ICUs in 28 hospitals in a large Midwestern metropolitan area during 1991 to 1997. Demographic and clinical data were collected from patients’ medical records and used in multivariable risk-adjustment models that examined the risk for in-hospital death and ICU length of stay.The adjusted odds of in-hospital death were 9% higher (OR 1.09; 95% CI, 1.04–1.15;P <0.001) for weekend admissions (Saturday or Sunday) than in patients admitted midweek (Tuesday through Thursday). However, the adjusted odds of death were also higher (P <0.001) for patients admitted on Monday (OR 1.09) or Friday (OR 1.08). Findings were generally similar in analyses stratified by admission type (medical vs. surgical), hospital teaching status, and illness severity. Adjusted ICU length of stay was 4% longer (P <0.001) for weekend or Friday admissions, compared with midweek admissions.Patients admitted to an ICU on the weekend have a modestly higher risk for death and ICU length of stay. However, the similar risk for death in patients admitted on Friday and Monday suggests that “weekend effects” may be more related to unmeasured severity of illness and/or selection bias than to differences in quality of care.