Lung injury is a medical entity that has been unrecognized for a long time. Even today, after ALI and ARDS have been defined, certain criteria still remain insufficiently explained. Due to the application of extracorporeal circulation (EKC), open-heart surgery has been frequently associated with ALI and ARDS, also known as "postperfusional lungs". Improved technical properties of the extracorporeal circulation and the possibility to perform surgery without its application have contributed to a decrease in the incidence rate of acute lung injury. This prospective study was performed and designed to establish the incidence of ACI and determine the onset of this disease in patients undergoing open-heart surgery with and without EKC. The study comprised 222 patients with heart disease involving several blood vessels who had neither associated disease of the heart and lungs nor diabetes mellitus of any kind and who were submitted to open-heart surgery. 134 patients had EKC and 88 did not. The determinant for proving ALI was the ratio PaO2/FIO2 of < 200. All respiratory parameters were measured, including (A-a)DO2, as well as all cardiac, blood vessel parameters and temperature during surgery and within the first 24 hours in the intensive care unit. All quantitative data with normal distribution between and within the groups were analyzed statistically using the parametric Student's t-test for independent and dependent samples. The quantitative data with abnormal distribution were tested using the nonparametric Mann Whitney U-test. The qualitative data were tested using the Chi square test. Differences in the values of (A-a)DO2 obtained by single measurements were tested using the variance analysis (ANOVA) with repeated measurements depending upon the patient group. For the prediction of ALI, a model of logistic regression and the ROC curve were designed. The relation between certain indicators was tested nonparametrically using the Spearman rang correlation. The statistical significance was accepted at the level of p < 0.05. The incidence of ALI in the EKC group was 68%. Using a model of logistic regression independent indicators for the development of ALI were detected. In addition to EKC, these included body weight and postoperative BE. The patients who underwent surgery with EKC had a 98% higher risk of ALI than the patients operated on without EKC. Each 5 kg of body weight increased the risk of ALI by 28.9%. Each preoperative increase in BE by 0.1 lowered the risk of ALI by 3%. The ratio PaO2/FIO2 of < 300 was found in 23% of the patients included into the study. They also had significantly increased (A-a)DO2 values, which is the actual proof of impaired gas exchange. As many as 98% of such patients were operated on using EKC. The application of EKC significantly increased the risk of ALI. The incidence of ALI was similar to the incidence confirmed by other studies that used the same determinants for proving ALI. Based on the results of this study it may be concluded that it is necessary to determine the level of (A-a)DO2 along with other respiratory parameters in order to adjust the respiration parameters and the percentage of inhaled oxygen and thus lower the risk of ALI and its consequences as much as possible. Further, it appears that reduction of body weight is one of the most important tasks for the patient undergoing open-heart surgery. It is also important to stress the conclusion of our study that patients should not be exposed to harming effects of a100% oxygen concentration if the ratio PaO2/FIO2 is higher than 300 and (A-a)DO2 is lower than 300%.