The aim of the investigation was to assess the functional state of the kidneys of preterm infants of various gestational ages with signs of acute kidney injury. Materials and Methods. The study included 30 preterm infants born at 29 to 36 weeks gestation with signs of acute kidney injury. Patients were divided into two groups: group I included children with low body weight, born at 3236 weeks of gestation; group II with very low birth weight, born at 2931 weeks of gestation. In the study of kidney function, the main markers were analyzed serum creatinine, urea, diuresis level, glomerular filtration rate (GFR) at the third week of life, as well as at discharge from the hospital when reaching the postconceptual gestational age of 36.0 [35.0; 39.0] weeks. Severity was assessed in both groups of patients. Results. In both groups of children, a slight degree of severity was recorded, the risk stage (Risk) according to the pRIFLE classification criteria (2007). The evaluation of the biochemical blood test did not show a statistically significant difference in creatinine, urea, GFR, and the rate of diuresis between groups of children with low body weight and very low birth weight. In both groups of patients, creatinine and urea levels in the third week of life exceeded the age standards. By the time the postconceptual gestational age was reached at 36.0 [35.0; 39.0] weeks, there was a statistically significant decrease in the level of these indicators in patients of both groups. In addition, it was found that premature children are at risk for the formation of hyperoxaluria (53% in the structure of crystalluria). Conclusion. Timely diagnosis of acute kidney injury in premature newborns will allow adequate therapy to prevent the progression and further formation of terminal renal failure. It is important to determine the level of excretion of oxalates in the urine to prevent the development of urolithiasis.