An earlier study of peak expiratory flow (PEF) in normal adults contained too few men aged over 55 and women aged over 65 for the regression equations to be used for prediction in older people. A subsequent study was therefore carried out on an additional 23 men and 29 women aged 55 or over who were lifelong non-smokers and satisfied the same strict criteria of normality that had been used in the original study. The data from both studies were combined and a new model used to calculate equations for the regression of PEF on age and height in the two sexes. With this model predicted values could be derived for men and women aged between 15 and 85. These new equations gave predicted values in men and women aged less than 55 and 65, respectively, which were almost identical with those reported previously. The new regression equations for PEF enable values to be predicted for people aged 15-85 and so enhance the accuracy of testing in the elderly.