['Survival from acute coronary syndromes and major trauma has been shown to depend on timely access to definitive treatment. We sought to identify the significance of intensive care unit (ICU) admission delay (lead-time) on the outcome of critically-ill medical patients with other diagnoses.', 'From 1 January 1997 to 31 December 2003, a prospective cohort study was performed in critically-ill patients requiring mechanical ventilatory support (MV) and/or renal replacement therapy (RRT), admitted directly to the Northern Hospital ICU within 24 hours of arrival in the emergency department (ED). Patients were excluded if, a) they were admitted following surgery, major trauma or transfer from another hospital, or b) their duration of ICU stay was < 8 hours. Data collected included de-identified patient demographics, final diagnosis, APACHE II mortality risk (pm) and lead-time (i.e. difference between times of entrance to the ED and ICU.) The primary outcome measure was hospital discharge status.', 'Six hundred and nineteen consecutive ICU admissions from the ED met the inclusion criteria and required MV (n = 557) and/or RRT (n = 162.) Non-survivors were older (median age 73 vs. 54 yrs) and sicker (median pm 0.72 vs. 0.23) compared with survivors. Multivariate analysis using logistic regression identified lead-time as a significant predictor of mortality (RR = 1.06 per hour, 95% CI =1.01 - 1.10; p=0.015) in addition to age, diagnosis and illness severity.'] Is survival of critically ill medical patients time-critical?