Nowadays still a great number of heart surgeries is usually carried out with the use of the system of cardiopulmonary bypass. The system replaces the function of heart and lungs during the surgery which enables it. The technique of the system for cardiopulmonary bypass is nowdays very modern and to a great extent is adapted to physilogical functions of our organism. Despite that cardiopulmonary bypass has unwanted effects on some physiological functions of our organism. One of the negative physiological effects of cardiopulmonary bypass is a possible kidney damage. The frequency of kidney failure which happens after a surgery with the use of cardiopulmonary bypass is a possible kidney damage. The frequency of acute kidney failure which happens after heart surgeries with the use of cardiopulmonary bypass is about 1-5 %. The frequency of subclinical kidney damage is bigger and by routine traditional clinical tests it can hardly be proved in earlier stages. The traditional tests, the so called tests for diagnostic kidney damage, are serumal urea and the clearance of creatinine from urine and the analysis of the sediments of urine. Initial tubulo-interstitial changes are discovered with graet difficulty. However, they can be discovered in greater damages. Subclinical kidney damage, especially kidney tubules, can be proved in early indicators of kidney damage. The two mentioned indicators are highly specific early indicators of the damage of kidney tubules – the protein of law molecular mass alpha-1- microglobulin (A1M) and lipocalin associated with neutrophil gelatinase (NGAL) which are determined in urine. This paper primarily studied unwanted effects of cardiopulmonary bypass upon kidney damage, that is to say, the function of proximal tubules of kidneys and the influence and role of hemodilution on renal function caused by „priming“ solution. In order to define kidney damage, early indicators of damaged kidney proximal tubules NGAL and A1M were determined in urine and for the sake of comparison they were determined in serum as well as traditional tests of urea and creatinine. This research showed the results of a prospective study which included 158 subjects who were subjected to elective heart surgeries with the use of cardiopulmonary bypass. The influence of hemodilution was observed in two groups. The first group of subjects with mild hemodilution was defined as the one where the ratio of hematocrits after a surgery in comparison with hematocrits during the surgery was greater than 60%. The second group of subjects with moderate hemodilution was the group where the ratio of hematocrits after the surgery in comparison with hematocrits during the surgery was less than 60 %. This research shows that moderate hemodilution during heart surgeries accompanied by cardiopulmonary bypass which is caused by diluted blood, the so called „priming solution“ of cardiopulmonary bypass, has an important effect upon the function of proximal kidney tubules. Early indicators of the damage of proximal kidney tubules A1M and NGAL in urine are very helpful and sensitive in proving disorders in early postoperative period of time after heart surgeries accompanied by cardiopulmonary bypass. The mentioned indicators have special values in observing dynamics during milder subclinical damages of proximal kidney tubules during cardiopulmonary bypass. A1M showed slightly greater sensitivity and increase of value than NGAL. The so called „classical“ indicators of kidney damage urea and creatinine in serum are not enough sensitive to prove early and subclinical kidney damage after heart surgeries accompanied by cardiopulmonary bypass. The study did not show, that in subjects with preoperative normal kidney function, transfusion treatment has a negative effect on the function of proximal kidney tubules during 24 hours after a heart surgery accompanied by cardiopulmonary bypass. The results of the study show that the length of cardiopulmonary bypass singificantly infleunces the damage of kidney function.