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Introduction: Renal cysts are a common disease that occurs at a rate of 7-10%. Currently there are no clinical recommendations for the treatment of patients with simple renal cysts. In the current literature there is some evidence that a simple renal cyst has negative effects on renal function. Decreased renal function occurs due to partial atrophy and loss of the renal parenchyma (in the "crater" area at the base of the cyst) caused by compression. Therefore, the efforts to analyze the effect of simple kidney cysts on kidney function and identify the characteristics of the cyst that affect renal function to determine the indications for surgical treatment remains a substantial task. The aim of the study was to analyze the effect of simple renal cysts on renal function, to investigate the relationship between cyst size, atrophied parenchyma volume, and renal function, and to determine indications for surgical treatment of simple renal cysts., Materials and Methods: We conducted a prospective cohort study. The study included 109 patients with simple renal cysts. Patients with a solitary cyst of the right or left renal kidney, grade I-II according to Bosniak classification, were included in the study. The estimated glomerular filtration rate (eGFR) of the patients was calculated using various formulas. A contrast CT scan of the urinary tract was also performed to determine the maximum size of the cyst, calculate the volume of the renal parenchyma, and the volume of the lost (atrophied) parenchyma. Patients underwent renal scintigraphy with calculation of total GFR and split renal function. We analyzed the symmetry of the function of both kidneys by comparing the GFR of the affected and healthy kidneys, analyzed the relationship between the presence of a kidney cyst and a decrease in GFR, between the maximum size of a renal cyst and a decrease in its function compared with that of a healthy kidney. We also analyzed the correspondence of total GFR values obtained in renal scintigraphy and GFR values calculated according to the formulas., Results: Data from 109 patients were available for analysis; the mean blood creatinine was 87.4 mol/L. The median maximum cyst size was 80 mm. The median baseline volume of the affected kidney parenchyma was 174 ml, the median volume of the lost parenchyma was 49 ml, and the median proportion of the lost parenchyma was 28%. The median total GFR was 77.07 ml/min. The median GFR of the healthy kidney was 45.49 mL/min, and the median GFR of the kidney affected by the cyst was 34.46 mL/min. The median difference in GFR of the healthy and affected kidney units was 11 mL/min and was statistically significant. Comparison of the eGFR values obtained by the formulas with the reference values of GFR obtained by scintigraphy showed that the Cockcroft-Gault formula with standardization on the body surface area calculated closest eGFR values to the reference ones. Correlation analysis revealed a statistically significant association between the proportion of lost parenchyma volume and the maximum cyst size: =0.37 with 95% CI [0.20; 0.52] (p-value = 0). A multivariate logistic regression model revealed that a statistically significant factor influencing the probability of a significant decrease in GFR was the percent of lost renal parenchyma volume (OR=1,13; =0)., Conclusions: Our study showed that growth of renal cysts associated with renal parenchyma atrophy and decrease of GFR of the affected kidney. An increase in the volume of atrophied parenchyma leads to the decrease in GFR of the affected kidney. The obtained data suggest that performing dynamic renal scintigraphy to assess the decrease in affected renal function and determine the indications for surgical treatment of renal cysts is a reasonable recommendation. According to the results of the study, the loss of 20% of the renal parenchyma can be considered an indication for renal scintigraphy. The Cockcroft-Gault formula with standardization on the body surface area allows to calculate closest GFR values to those obtained by scintigraphy and, therefore, can be recommended as the optimal formula for calculating eGFR in daily clinical practice.