ObjectiveAlthough admission of patients to a medical ward after 5:00 pm has been associated with increased mortality rate and possibly shorter hospital stay, the association between timing of admission to the intensive care unit and outcome has not been studied. The objective of this study was to determine whether there are any associations between the timing of patient admission to a medical intensive care unit and hospital outcome.DesignA retrospective cohort study that used an Acute Physiology and Chronic Health Evaluation III database containing prospectively collected demographic, clinical, and outcome information for patients. Patients were divided according to the time of admission into daytime (from 7:00 am to 5:00 pm) and nighttime admissions. We further subdivided nighttime admissions into two groups (regular and heavy workload) according to the number of patients who were admitted during the same shift.SettingMedical intensive care unit (a 15-bed unit in an academic referral hospital).Patients6,034 patients consecutively admitted to our medical intensive care unit over a 5-yr period starting April 10, 1995.InterventionsNone.Measurements and Main ResultsThe patients admitted at night had a lower mortality rate (13.9 vs. 17.2%,p< .0001), adjusted for admission source and severity of illness. Their hospital stay was shorter, 11.0 days ± 13.5 (median 7) vs. 12.7 ± 14.8 (median 8;p< .0001), as was their intensive care unit stay, 3.5 ± 4.4 days (median 2) vs. 3.9 ± 4.7 (median 2;p< .0001), compared with the daytime admission group. The nighttime shifts that admitted three or more patients (heavy workload) had the same mortality rate (13.2%) as those with fewer admissions (14.5%;p= .5961). Hospital and intensive care unit stays were also similar in both workload groups.ConclusionsNighttime admission to our intensive care unit is not associated with a higher mortality rate or a longer hospital or intensive care unit stay compared with daytime admission.