Introduction: Outcomes following in-hospital cardiac arrest (IHCA) are generally poor though different patient populations may benefit to different degrees from admission to Intensive Care Units (ICUs). Risk stratification algorithms may be useful in identifying patients who are most likely to benefit from ICU admission and so may aid allocation of this scarce resource. We aimed to compare the performance of the Acute Physiology and Chronic Health Evaluation II (APACHE II) and Intensive Care National Audit and Research Centre (ICNARC) scoring systems in predicting outcome following ICU admission after IHCA in younger (≤69 years) and older (≥70 years) patients. Materials and Methods: We performed a retrospective observational study in two adult ICUs from January 2006 to February 2010 inclusive. Patients were divided into younger (≤69 years) and older (≥70 years) patients. The primary outcome measures were acute hospital mortality and area under the curve (AUC) calculation for receiver operating characteristic (ROC) analysis. Results: Two hundred and sixty-one adult consecutive adult patients admitted following IHCA. Hospital mortality was 58.6%. ROC analysis demonstrated that ICNARC was more accurate than APACHE II in predicting acute hospital outcomes in the adult population (AUC 0.734 vs. 0.706). Both scoring systems performed weaker when predicting outcomes in younger patients compared to older patients (ICNARC AUC 0.655 vs. 0.810; APACHE II AUC 0.660 vs. 0.759). Discussion: Both APACHE II and ICNARC predict outcome well in older patients. In younger patients, their value is less clear, and so they must be used with caution.