Acute lymphoblastic leukemia (ALL) is the most common malignant disease in children under 15 years, and its frequency is 25%. Long term survival is achieved in more than 80% of the cases, but because of the supression of the imunological sistem the children have frequent infections. The aim of this study was to analyze the frequency, type and time of occurence of infections in children treated for ALL. The study included 62 children treated for ALL at the Division of Hematology and Oncology, Department of Pediatrics, School of Medicine and University Hospital Center Zagreb. The treatment was divided into five phases – induction, early intensification, consolidation, reinduction and maintenance. The most infections were caused by coagulase-negative Staphylococcus, followed by Pseudomonas aeruginosa, Streptococcus viridans, Escherichia Coli, Enterococcus faecalis and Klebsiella pneumoniae. The greatest number of infections occured during the consolidation and reinduction phases. Bacterial infections are a frequent cause of mortality in children treated for acute lymphoblastic leukemia. The spectrum of bacteria causing infections in this group of patients is changing rapidly and they are becoming resistand to many antibiotics. Because of that it is necessary to frequently adjust the guidelines for infection treatment in immunosupressed patients. The proportion of infections caused by gram-positive bacterias is rising because of the agressive protocols for chemotherapy and the use of intravascular catheters. It is important to abide to preventive measures in patient care, to maintain a good hygiene of the skin, oral cavity, anal region and the surrounding of the patient.