Background: Over 300 million individuals worldwide suffer from asthma. Furthermore, during the past three decades, western nations have demonstrated increased prevalence. 7.7% of people in Egypt's Nile Delta were found to have bronchial asthma. Follistatin-like 1 (FSTL1) may be crucial for the respiratory system, according to a number of studies. FSTL1 controls the maturation of alveoli, cartilage, and the development of the lungs. Aim: To evaluate follistatin like protein 1 level in the serum of pediatric patients with asthma, and determine its association to asthma severity. Patients and methods: This case-control study was carried out on 60 children at pulmonology unit in pediatric department and clinical pathology department, Zagazig University Children Hospital in the period from May 2020 to June 2022. They were divided into group A: Consisted of 45 asthmatic children subdivided into 3 groups (mild – moderate – severe) and group B: consisted of 15 age and sex matched healthy children. Results: There was highly statistical significant increased FST level in severe group when compared with moderate group, mild group and control group. Conclusion: In asthmatics, plasma FSTL1 levels were higher and positively correlated with severity of asthma.