BACKGROUND: Community Acquired Pneumonia (CAP) is a common infection which is associated with a significant mortality. Shock index, heart rate divided by blood pressure, has been shown to predict mortality in several conditions including sepsis, acute myocardial infarction and traumatic injuries. Very little is known about the prognostic value of shock index in community acquired pneumonia (CAP). OBJECTIVE: To examine the usefulness of shock index (SI) and adjusted shock index (corrected to temperature) (ASI) in predicting mortality and hospital length of stay in patients admitted to hospital with CAP. METHODS: A prospective study was conducted in three hospitals in Norfolk & Suffolk, UK. We compared risk of mortality and longer length of stay for low (=1.0, i.e. heart rate > systolic BP) SI and ASI adjusting for age, sex and other parameters which have been shown to be associated with mortality in CAP. RESULTS: A total of 190 patients were included (males=53%). The age range was 18-101 years (median=76 years). Patients with SI & ASI >1.0 had higher likelihood of dying within 6 weeks from admission. The adjusted odds ratio for 30 days mortality were 2.48 (1.04-5.92; p=0.04) for SI and 3.16 (1.12-8.95; p=0.03) for ASI. There was no evidence to suggest that they predict longer length of stay. CONCLUSION: Both SI and ASI of >1.0 predict 6 weeks mortality but not longer length of stay in CAP.