Background: Asthma among obese represents a unique phenotype. Mast cells are more abundant in obese. Serum tryptase (ST) is a marker of mast cell numbers or activity. Since obesity and asthma have been linked in epidemiological studies, a possible higher mast cell activity in obesity could be a factor between the two conditions. This study was to investigate ST and its potential association between obesity and childhood asthma.Methods: Study recruited 60 asthmatic children, their age ranged from 5-16 years. They were divided according to BMI centile to 30 obese and 30 non-obese asthmatics. Thirty healthy non-asthmatic, nonatopic and non-obese children; were included as a healthy control. Serum tryptase, atopy (skin prick test reactivity) and spirometry were assessed.Results: Frequency of atopy and positive skin prick test signifi cantly increased among obese more than non-obese asthmatics (P<0.05, OR = 1.96, 95% CI = 1.27-3.24). FEV1% of predicted mean levels were lower among obese than non-obese asthmatics (p<0.05). ST was signifi cantly higher in asthmatics than in controls with a mean ±SD of 53.3±13.78 ng/ml and 10.06±4.39 ng/ml respectively. ST was higher in obese than non-obese asthmatics with a mean ±SD of 71.73±19.17 ng/ml and 34.5±8.68 ng/ml respectively (P<0.05). There was a negative correlation between ST and FEV1 % of predicted and positive correlations between ST and age, BMI, and waist circumference among asthmatics. Conclusion:Mast cells play a role in both obesity and asthma, serum tryptase, being a marker of mast cell activation, could represent a link between them.