Body mass index (BMI) is a significant indicator of health and well-being.The notion of considering BMI as a vital sign was first put forward over a decade ago; however, many health-care professionals do not routinely measure height and weight, or calculate BMI. Given the current obesity epidemic, this oversight has implications for the diagnosis and management of obesity and for appropriate healthcare delivery for obese patients. The authors propose that BMI should be considered a vital sign.They suggest that protocols be put in place to encourage health-care providers to record BMI routinely and to take action to address a BMI that is outside the normal range. Promoting the routine calculation of BMI on admission or initial assessment may encourage health-care professionals to become more aware of obesity and its impact on the patient and the health system.