Background: The Childhood Asthma Management Program study revealed that 25.7% of children with mild to moderate asthma exhibit a loss of lung function. The objective was to assess the trajectories of function by means of serial FEV1 in asthmatic children participating in out-of-hospital follow-up. Methods: A total of 295 children (199 boys) who had undergone at least 10 spirometry tests from the age of 8 were selected from a single-center open cohort. The annualized rate of change (slope) for prebronchodilator FEV1 (percent predicted) was estimated for each participant and three patterns were defined: significantly positive slope, significantly negative slope, and null slope (non-significant P-value in the Pearson test). The standard deviation (SD) of each individual slope was recorded as a variability criterion of FEV1. Results: The median (25th and 75th percentile) age at inclusion and the last visit was 8.5 (8.2, 9.3) and 15.4 (14.8, 16.0) years, respectively. Tracking of function (null slope) was observed in 68.8% of the children, while 27.8% showed a loss of function (negative slope) and 3.4% showed a gain in function (positive slope). The children characterized by loss of function depicted a better initial function and a lower FEV1 variability during their follow-up than children with tracking or gain of lung function. At the last visit, these children were characterized by a lower lung function than children with tracking or gain of lung function. Conclusion: Children with a better initial FEV1 value and less FEV1 variability are more prone to loss of lung function.