Asthma is a common disease in children with grief consequences on social and educational performance of sufferer. Montelueukas has been a mainstay drug for asthma attack controller medication. However, its effect on blood is not well defined. The main objective is to determine the effect of the drug on white blood cells. This case control study encompasses 72 children with confirmed asthma, 36 of them received 4mg of monteleukast granule daily for four months, the remaining half did not receive the drug. Complete Blood Count (CBC) was performed at the beginning of the study and then monthly till the end of the study. There was no statistically significant difference in the CBC parameters at beginning and during the first two months (P>0.5) between the two group. There was a trend toward a reduction in the lymphocyte count in the treated group in the third month in comparison to the control one. The last CBC measurement (4 months) showed statistically significant reduction in the lymphocyte count (P<0.01) in the treated group when compared to the control group. In conclusion: Monteleukast negatively influence the number of lymphocytes and caution should be exercised to avoid prescribing the drug in conditions known to reduce lymphocyte counts such as Covid-19 infection.