Introduction: Asthma is a chronic inflammatory disorder of the airways characterized by a predominant Th2 response. Thymic stromal lymphopoietin (TSLP) has been considered to switch on the T2 response in asthma. ----- Aim: To evaluate the serum levels of TSLP in children with asthma and healthy controls, and to establish the correlation of serum TSLP with specific clinical and laboratory characteristics of our patients. ----- Patients and methods: The study included 207 children with asthma and 100 healthy controls aged 1 to 13 years. Serum levels of TSLP were measured in all children by enzyme-linked immunosorbent assay (ELISA) using the Human TSLP ELISA Kit (abcam, ab155444). In 23 children with asthma, TSLP were measured in acute asthma exacerbations as well. Results: Serum levels of TSLP were significantly higher in children with asthma compared to healthy children (p<0,005). No statistically significant difference was found in serum levels of TSLP between the three different asthma phenotypes (asthma with allergic sensitization, asthma without allergic sensitization and virus induced asthma), nor between children with well-controlled and uncontrolled asthma (based on asthma control test score in children older than 4 years or spirometric measurements). Serum level of TSLP is not a good indicator of pulmonary function, nor is it significantly elevated in acute asthma exacerbation. Antiinflammatory therapy and disease severity do not affect serum levels of TSLP. A statistically significant differences was found between the quartile groups of TSLP depending on body mass index (p<0.05), and the role of adipose cells on serum levels of TSLP is possible. Slimmer children have lower, while obese ones have higher serum TSLP values. ----- Conclusion: The results of this study confirmed the hypothesis that serum levels of TSLP in children with asthma are significantly elevated than in healthy children, but serum TSLP is not a good biomarker in routine diagnostics because it does not help with classifying children into different asthma phenotypes, neither in the assessment of childhood asthma control, nor is it a good indicator of lung function. The question of correlation between serum levels of TSLP and asthma in obese individuals remains open.