OBJECTIVE To observe the alterations of pulmonary function in infants with respiratory diseases. METHODS A total of 900 infants with respiratory diseases were recruited and pulmonary function measured in 30 healthy infants. The tests were performed in the sleeping infants with sedation. Tidal breathing flow-volume (TBFV) loops were recorded when infants were breathing quietly. Passive flow-volume technique was used to obtain static respiratory system compliance and resistance. Functional residual capacity was measured by body plethysmograph. RESULTS The TBFV loop showed proximate round or oval curve in healthy infants. The shape and parameters of TBFV loop had significant differences in infants with respiratory diseases as compared with healthy controls. The TBFV loop displayed a concave expiratory curve and ratio of time to reach tidal peak flow to total expiratory time, the expiratory volume till peak flow divided by the total expiratory volume significantly decreased in infants with small airway obstruction. The expiratory or inspiratory curve showed a plateau and the ratio of mid-expiratory to mid-inspiratory flow was less than 60% or over 150% in infants with upper airway obstruction. The TBFV loop turned narrow and lung volume decreased in infants with restrictive diseases. CONCLUSIONS The TBFV loop show proximate round or oval curve in healthy infants. Pulmonary function has significant differences between healthy controls and infants with respiratory diseases. Pulmonary function test is useful in the assessment of respiratory diseases.