BACKGROUND: The concavity of the descending limb of the maximum expiratory flow-volume loop (MEFVL) is the earliest change associated with airflow obstruction in small airways (ATS/ERS Task Force). The shape of the MEFVL changes with age but there are no reference values for shape indices for preschool and school children. OBJECTIVE: To define pediatric reference values for spirometric data and 3 shape indices of MEFVL: 2 geometric indices: the β angle i.e., the angle between the first ½ part and the 2nd part of the MEFVL and the forced expiratory flow after 50% of the forced vital capacity (FVC) has been exhaled/peak expiratory flow (FEF50 /PEF) ratio; and a ratio that describes relative growth between airway and lung parenchyma, the forced expiratory flow between 25 and 75% of FVC/FVC ratio (FEF25-75 /FVC ratio). METHODS: Data were obtained from 446 Caucasian children (2.5 to 15-year-old). The lambda, mu, sigma method was applied. RESULTS: References for spirometric parameters and 3 shape indices. The geometric indices decreased with age from 3 years of age (mean β angle was 215° and FEF50 /PEF ratio was 0.82) until 8 years of age (mean β angle was 191° and FEF50 /PEF ratio was 0.60) and then remained constant. The FEF25-75 /FVC ratio also decreased with age. Sex was a significant determinant for FEF25-75 /FVC ratio predicted values. CONCLUSIONS: This study provides standard reference equations for indices of mid-expiratory flows in children and we suggest using the FEF50 /PEF index.