Most acutely unwell patients entering the hospital through either the emergency department or through an emergency assessment unit will have pulse oximetry performed as part of their initial clinical assessment. Although many GPs now have access to a pulse oximeter, pulse oximetry tends to be used much less frequently in primary care, and mainly to assess patients with respiratory conditions or symptoms. This article shows how pulse oximetry can be used as a screening tool during the initial assessment of acutely unwell patients to detect unanticipated hypoxia, as well as to identify those patients with respiratory symptoms who are beginning to decompensate.