The incidence of pancreatitis in patients with haematopoetic neoplasms who are treated with L-asparaginase is fom 2 to 24%. In majority of cases the pancreatitis is oedematous and self-limiting. Acute haemorrhagic or necrotizing pancreatitis caused by L-asparaginase is rare but potentially life-threatening complication. We present 2 cases of acute pancreatitis in children aged 2 and 4 years. They were diagnosed to have acute lymphoblastic leukaemia and were treated according to the ALLLIC BFM 2002 protocol. Acute pancreatitis developed in these children after induction therapy and was followed by formation of a pseudocyst. In both cases the diagnosis of this complication was made directly after phase I of the protocol I (after eighth dose of L-Asparaginase). In the first case the course of acute pancreatitis was mild. Normalization of the amylase levels occurred after 7 days and the diagnosis of post inflammatory cyst was made 15 days after the first signs of the disease. But thereafter, during the additional complication (pneumonia with Pseudomonas aeruginosa bacteriemia) the pancreatic cyst became infected. In the second case acute pancreatitis had a severe course and the child required treatment in the Intensive Care Unit for 21 days. The cyst was diagnosed after 20 days from the beginning of symptoms. The surgical procedure, applied in both cases was internal drainage by anastomosis of the cyst with the back wall of the stomach. Antileukaemic treatment was recommenced after 6-8 weeks when complications resolved. Currently both children are well and remain in haematological remission and continue maintenance chemotherapy.