Objective measurement of pulmonary function is important whenever possible not only to confirm the clinical diagnosis but to monitor asthma as well. Patient's subjective symptoms and doctor's subjective assessment correlate poorly with pulmonary functions and declines in pulmonary functions may predate acute deteriorations in asthma. This has been stressed in the new guidelines for management of asthma. Expiratory spirometry should be used as soon as the child is old enough to cooperate. Peak flow monitoring and pulmonary function measurements can generally be done by age 6 or 7 years and in some children peak flow measured as young as 3 to 4 years old.