PURPOSE: Preschoolers complete forced expiration in a short time, sometimes more quickly than in 1 second, and therefore the importance of forced expiatory volume in 0.75 seconds (FEV0.75) or forced expiatory volume in 0.5 seconds (FEV0.5) has been raised. The purpose of this study is to evaluate the clinical usefulness of DeltaFEV0.75 and DeltaFEV0.5. METHODS: We analyzed 401 subjects of an asthma group, and 150 subjects of a control group under 7 years of age. RESULTS: DeltaFEV1, DeltaFEV0.75 and DeltaFEV0.5 values of the asthma group were significantly higher than those of the control group, respectively (P<0.0001). DeltaFEV1 (0.60; 95% confidence interval [CI], 0.57 to 0.62), DeltaFEV0.75 (0.61; 0.58 to 0.65), and DeltaFEV0.5 (0.60; 0.56 to 0.64) showed no significant difference in the diagnostic ability of asthma when airway reversibility is defined as DeltaFEVt> or =12%. Cutoff values for asthma were 8.6% in DeltaFEV1, 7.9% in DeltaFEV0.75 and 14.2% in DeltaFEV0.5. DeltaFEV0.75 (0.91; 0.88 to 0.94) showed significantly higher area under curve (AUC) than DeltaFEV0.5 (0.77; 0.73 to 0.82) when stratified by 12%, in predicting airway reversibility defined as DeltaFEV1> or =12%. Cutoff values were 12.3% in DeltaFEV0.75, and 13.4% in DeltaFEV0.5. When airway reversibility is defined as DeltaFEV1> or =8.6%, DeltaFEV0.75 (0.90; 0.87 to 0.92) also showed significantly higher AUC than DeltaFEV0.5 (0.79; 0.75 to 0.82), and Cutoff values were 8.4% in DeltaFEV0.75, and 11.3% in DeltaFEV0.5. CONCLUSION: DeltaFEV0.75 or DeltaFEV0.5 can be a means to replace DeltaFEV1 for diagnosis of asthma and assessment of airway reversibility in preschool children.