The study objectives were to examine the differences between Peak Expiratory Flow (PEF) formulae in the literature and to assess the potential impact of those differences on the interpretation of clinical guidelines for asthma management. We calculated 100% PEF values for hypothetical patients at the 50(th) percentile for height and weight and classified the percent predicted PEF into severity groups according to national asthma guidelines. Choosing different formulae could give an 18 year old man a 100% predicted PEF as low as 501 L/min and as high as 730 L/min (delta = 229 L/min); and a 35 year old woman a classification of severe (46%) using one, but moderate (57%) using another. Predicted PEF varied widely across formulae and choice of formula may alter guideline-based care. We propose recently published population-based equations as the reference standard for future asthma guidelines.