Introduction. The choice of surgical treatment with kidney stone disease directly depends on the size of the kidney stones. According to clinical guidelines for the treatment of urolithiasis in children, the size of a kidney stone in adult patients is used to determine tactics. Normally, the longitudinal size of the kidney of a child, for example, 1-3 years old, is 2 times smaller than that of an adult, and the width of the child's ureter is 37% less than the width of the adult's ureter. But anatomical and age differences in children (infant, preschool, school and adolescence) are not taken into account. Existing prognostic nomograms for urolithiasis (Dogan, Onal, CMUN, SKS, Guy's Stone Score, S.T.O.N.E, and CROES) that are validated for use in children also do not take this into account. Thus, the size of a kidney stone, as a criterion for choosing an effective and safe method of surgical treatment, needs to be clarified. The purpose of the study: to define the concept of a large kidney stone in children of different age groups. Materials and methods. We analyzed clinical guidelines, domestic and foreign publications on the use of «kidney stone size» in children of different ages and the principles of substantiation of the term large kidney stone. 320 histories of percutaneous nephrolitholopaxy (PNL) from 2008-2019 were retrospectively and perspectively analyzed. aged from 1 to 17 years, the average age of patients is 6.6 years. The average calculus size was 26 (15-58) mm. Number of patients in different age groups: toddlers 1-3 y/o – 73, pre-school child 3-5 y/o. – 71, school child 6-12 y/o – 79, adoloscents 12-18 y/o – 97. By type of calculus: single –125 (39.1%), multiple – 68 (21.3%), K1, K2 – 46 (14.3%), K3, K4 – 81 (25.3%). The weight of the patients ranged from 8–94 kg, the average weight was 21 kg, the height was from 73–180 cm, and the average height was 120 cm. The length of the child's kidney depends on his age, the formula for calculating the length of the kidney (cm) = 6.79 + 0.22 x age (g)) Result. KSS-CDA: Kidney stone size in children different ages was proposed: KSS-CDA = (stone size (mm)/the kidney length (mm))*100%. KSS-CDA makes the interpretation of the size of kidney stones in children from different age groups objective. The definition of "Large" kidney stone is used if the size of the stone is 20% or more of the longitudinal size of the kidney in children. For example, a child is 1 year 7 months old, the stone is 15 mm, the length of the kidney is 69 mm. The SSEF is 21.74%, this kidney stone can be described as «large». The «medium» stone size is 10-20% (6-14 mm), small