The aim of the study was to determine the side effects of asparaginase administration during treatment protocol for childhood non-Hodgkin's lymphoma (NHL). Drug adverse reactions occurred in 20/66 of patients (30,3%) treated in 9 centres in Poland between 1993 and 1998. The most common side effects were coagulation disturbances in 12/66 of the children (18,2%), which occurred due to reduced production of important coagulation factors. Six patients (9,1%) developed impairment of liver function (9,1%). Drug toxicity caused the modifications of treatment protocol in 12/66 (18,2%) of patients, mainly in the induction phase; 3 children died due to relapse of disease.