The study aims to identify the reasons for, and outcomes from, unplanned transfers from subacute care to acute care. A retrospective patient record review of patients requiring unplanned transfer from subacute to an acute care emergency department (ED) from 1 July 2008 to 30 June 2009 was undertaken. Data collected included patient demographics, clinical characteristics in preceding transfer, and on ED arrival and outcome data. There were 136 patients included in the study with a median age of 81 years. The most common reasons for transfer were respiratory problems and altered conscious state. In the 24 h preceding transfer, 92.6% of patients had ≥ 1 physiological abnormality and 10.3% of patients had no physiological parameters documented. On ED arrival, 75% of patients had physiological abnormalities. Hospital admission occurred in 75% of patients and the inpatient mortality rate was 14.7%. Factors associated with inpatient mortality were tachypnoea and severe hypoxaemia in 24 h preceding transfer and tachypnoea, hypoxaemia, hypoxaemia, severe hypoxaemia and hypothermia on ED arrival. Patients requiring unplanned transfer had higher inpatient mortality than older hospital users. Reasons for unplanned transfer reflect known predictors of in-hospital adverse events so predictive use of physiological data and patient characteristics might optimize patient safety.