L-asparaginase (L-ASPA) is one of the most important drug in the treatment of acute lymphoblastic leukemia (ALL). The aim of the study was to asses usefulness of ammonia (direct product of the reaction catalyzed by L-ASPA) concentration determination in monitoring of L-ASPA treatment. Patients and methods: Eighty seven children with ALL were enrolled to the study. L-ASPA activity, ammonia concentration and parameters assessing function of liver, kidney, pancreas and coagulation system were determined. Results: Most patients (81%) had therapeutic L-ASPA activity. Positive correlation between average ammonia level and average L-ASPA activity and negative correlation between ATIII activity and L-ASPA activity were found. On the basis of the ROC curves analysis of the utility of ammonia concentration and ATIII activity determination in indirect L-ASPA activity evaluation was revealed. Allergic reaction was observed in 49% of patients, more often in patients with L-ASPA activity below 100 IU/l (78%) and with undetectable activity (100%). Conclusions: Determination of ammonia concentration and ATIII activity can be useful in indirect L-ASPA activity evaluation. In patients with low and especially undetectable L-ASPA activity or low ammonia concentration and/or high ATIII activity, change of L-ASPA preparation might be considered to avoid hypersensitivity reaction and assure therapeutic activity of the drug.