Altered secretion of saliva and its changed composition in patients undergoing chemotherapy often results in oral mucosa pathologies. The aim of this study was to assess the impact of oral hygiene and the concentration of calcium, phosphate and magnesium ions in the saliva on developing oral mucositis in children with acute lymphoblastic leukaemia (ALL) during anticancer therapy. The study included 78 children, aged from 2 to 18 yrs suffering from ALL on anticancer therapy. The results of biochemical scores found that the concentrations of calcium, phosphate and magnesium ions in the saliva of children with ALL were affected. Oral hygiene condition and lower concentrations of selected biomarkers in the saliva in children with ALL during chemotherapy may affect the intensity of the pathological changes in the oral mucosa.In the group of children examined unstimulated saliva was collected in the morning two hours after a meal to determine the concentration of calcium, phosphorus and magnesium ions. The samples of collected saliva were centrifuged for 15 min at 5,000 rot/min. The centrifuged saliva samples were frozen at -80°C, until the time of biochemical scores.The concentration of calcium ions was determined by the colorimetric method of Arsenazo III at neutral pH, the complex was read at 660nm wavelength with the use of reagent kit Konelab™ CALCIUM, Thermo Electron Corporation.The concentration of phosphorus ions was determined calorimetrically by the direct method without deproteinization by measuring UV absorption of the resulting phosphorus-molybdenum complex formed in the acidic environment/absorption at 340 nm wavelength is proportional to the concentration of phosphate ions in the material tested. The measurement was done using reagent kit Phosphorus UV BioMerieux.The concentrations of magnesium ions were determined by colorimetric method without deproteinization, using calmagite as chromogen and the reagent kit Mg-Kit, BioMérieux.Microelements were determined on a biochemical analyzer Konelab Kone Pro.The results were analyzed statistically. The measurable parameters were presented as mean, Median, minimum and maximum values and standard deviation. Mann-Whitney U test was used to compare 2 independent groups, Spearman's Rank Order Correlation test was used to analyze the relationship between two quantitative parameters;