OBJECTIVE: Dry powder inhalers (DPIs) are increasingly being used to deliver drugs for the treatment of asthma. It is known that DPIs require a crucial minimal inspiratory flow. Previous studies have demonstrated that the peak inspiratory flow (PIF, L/min) through a DPI is dependent on the type of device, the age of the patient, and the level of bronchial obstruction. However, the peak inspiratory flow of healthy preschool children in China remains scant in the literature. The present study aimed to analyze the ability of inspiring flow through the resistance state of ordinary use inhaler in Shenzhen healthy preschool children by measuring the peak inspiratory flow through the different analogue dry powder inhalers and go further into the relationship between it and the age, weight and forced expiratory volume of the children. METHOD: A survey in 370 healthy preschool children aged 3 to 6 years (75 children aged 3 years, 104 children aged 4 years, 100 children aged 5 years and 91 children aged 6 years) was carried out in Shenzhen. Peak inspiratory flow (PIF) was measured without and with resistances, which mimicked the internal resistances of several inhalers, Diskus, Turbuhaler, Autohaler, Surehaler by PIF meter (In-check DIAL) and then data PIF-N, PIF-D, PIF-T, PIF-A and PIF-S were obtained. Peak expiratory flow (PEF) was measured by PEF meter (MicroPeak, USA). These two measurements were made in a well-controlled setting, and at least three attempts were recorded to establish maximum achievement. Six spirometry parameters forced vital capacity (FVC), forced expiratory volume at 0.5 second (FEV 0.5), forced expiratory volume at 0.75 second (FEV 0.75), forced expiratory volume at one second (FEV1), maximal mid expiratory flow rate (FEF 25 - 75, PEF were measured by using COSMED spirometry of Italy and the FVC measurements should be around the quality control for spirometry in preschool children which we suggested and published in 2005. All data were expressed as mean +/- SD and analyzed with the statistical software SPSS 12.0 for Windows. Pearson's test was used for calculation of the significances of the correlation coefficients. Variance analysis was used for analysing the variability of inspiratory flows through the inhalers. RESULTS: Results were obtained from 295 children aged 3 - 6 years who successfully finished the tests. The PIF-N, PIF-D, PIF-T, PIF-A and PIF-S were significantly different among the groups aged 3 yrs, 4 yrs, 5 yrs and 6 yrs. The peak inspiratory flow significantly increased with age. The PIF-N, PIF-D, PIF-T, PIF-A and PIF-S in the children of 110 cm height and above were significantly higher than those in the children below 110 cm height, so were the parameters between the children of 120 cm height and above and the children below 120 cm. PIF correlated significantly with age, height and weight and the Pearson coefficient was 0.3 - 0.5. The PIFs in different inhalers varied because of the different inner resistances. The minimum and optimum PIFs in resistances of Diskus, Autohaler and Surehaler could be achieved in almost all subjects, but those in resistances of Turbuhaler could be achieved in only 87.5% subjects, most of whom aged 3 yrs or below 100 cm height. There were good correlations between the PIFs in different resistances and main parameters of ventilation function (FVC, FEV 0.5, FEV 0.75, FEV1, FEF 25 - 75, PEF), PEF was the best among them (Pearson correlative coefficient was 0.6). CONCLUSION: The inspiratory ability of the children can be predicted and assessed by using routine measurement of lung function of normal pre-school children. As to the pre-school children of varying ages, the variety of inspiratory ability should be considered completely in the selection of inhaler used during the treatment. The best inhaler suitable for them should be selected properly in order to obtain the best efficacy of treatment individually.