The Methtrexateoncology drug has a significant role in treatment of acute lymphoblastic leukemia. Although this effective role of Methotrexate, it has many adverse effects especially if administered in high dose. To estimate of the serum level of Methotrexate and clearance, pharmacokinetic and pharmacodynamic of Methotrexate. A clinical prospective follow up study conducted in Baghdad Hematology center of Medical city during the period from 1st of November, 2018 to 20th of April, 2019on sample of 53 Iraqi patients with hematological malignancy. The diagnosis of acute lymphoblastic leukemia or non-Hodgkin's lymphoma was done by the physicians through proper investigations. The Methotrexate dose and infusion rate were designed by the responsible Hematologist. The patients were followed through serum collection and examination of Methotrexate level by Apput Machine. The Methotrexate adverse effects were present in 6 (11.3%) patients with hematological malignancy; 55% mucositis, 18% hepatic adverse effects, 18% renal adverse effects and 9% central nervous system side effects. Mean serum Methotrexate 24 hr for patients with hematological malignancy was significantly higher among patients with positive hepatic, renal and central nervous system adverse effects (p<0.05). Mean serum Methotrexate 72 hr for patients with hematological malignancy was significantly higher among patients with positive hepatic and renal adverse effects (p<0.05). The common Methotrexate adverse effects after 24 hours and after 72 hours in treatment of hematological malignancy in patients are hepatic, renal and central nervous system adverse effects.