Background:Patterns of early decline in lung function have been reported in pediatric asthma. Our main objective was to identify pulmonary function trajectories in spirometry, from preschool age to school age. Methods: Prospective study conducted between 2016 and 2021. Patients with persistent asthma who underwent IOS-spirometry at baseline and after 3 years were recruited. Abnormal spirometry was defined according to ATS/ERS guidelines. Methods: X² and ANOVA was used to compare clinics characteristics and means of IOS-spirometry parameters between trajectories. Results: 86 patients with a mean age of 5,3 and 8,3 years in their first and second evaluation. 58,2% were male. Between both evaluations, 70,9% of the patients maintained normal spirometry (Track 1), 9,3% had abnormal preschool spirometry than normalized at school age (track 2), and 19,8% had abnormal preschool spirometry, which remained abnormal at school age (track 3). Track 3 patients had a lower average birth weight (2,4k vs 3,02k p=0,04), higher average of exacerbations (5,3 vs 2,01 p=0,00002), higher average of hospitalizations (0,61 vs 0,16 p=0,04), lowest average parameters in spirometry (FEV1/FVC % ratio, FEV 0,75/FCV % ratio, FEV 0,75 L, FEV 0,5L), averages significantly lower in X5 Kpa/Ls and higher in AX Kpa/Ls, that those in trajectory 1. Conclusions: Trajectory 1 was the most common, with persistent normal lung function. Trajectory 3, the second most frequent, started follow-up with decreased lung function in spirometry and small airway disfunction in the IOS that were maintained at school age. Children who followed trajectory 3 had lower birth weight, more exacerbations and hospitalizations than children in trajectory 1.