INTRODUCTION The diffusion capacity for carbon monoxide (DLCO) provides a measure of gas transfer in the lungs (1). Swimmers have larger lungs and possess better diffusing capacity than other athletes and controls (2,3), but none study has measured the possible acute modification in lung diffusion of elite swimmers after training session. During the last decade, it has been described the presence of swimming-induced pulmonary oedema (SIPE), an uncommon occurrence which it is usually presented during strenuous long-distance swimming, altering the DLCO (4). The aim of this study is to evaluate the modifications in the pulmonary alveoli-capillary diffusion provoked by swim training over a period of time. METHOD The participants are 21 international competition swimmers (14 men and 7 women) from 13 to 19 years old with a training schedule of 25-30 hours of training per week. The lung diffusion changes have been measured before and after 11 training sessions. The water temperature of the pool is 27°C. The material used is a computerized spirometer (Gashorn, PowerCube Diffusion+, Niederlauer, Germany). The single-breath method was used to measure the DLCO(5). Respiratory parameters were analysed using a multifactorial analysis of variance (ANOVA) and the software used to facilitate the statistical analysis was StatGraphics 18. RESULTS The main finding of this study is that the pulmonary diffusing capacity is diminished after the swimming training in a follow-up condition (44,4±8,4 vs. 43,3±8,4 p<0,05). In addition, men have higher values than women (47,3±6,2 vs. 37,2±8,9 p<0,05) and all of them showed higher values of diffusion at the afternoon compared to the morning session (44,9±8,4 vs. 43,2±8,5 p<0,05). DISCUSSION The stress of exercise on the pulmonary system could reveal subtle changes in the permeability of the lungs which, normally, may not be an inconvenience in the extreme-developed lungs from elite athletes. Besides there are additional factors, not evaluated in this study, which could provoke that swimmers develop a type of alteration on diffusion capacity such as horizontal form of locomotion, high core blood flow and arterial pressure, cold water temperature (if they train outdoor), long-term exercise duration (4) and even a possibly inflammatory collateral process related to chlorine, or swimmers asthma. This study shows that elite swimmers could develop repeated and subclinical SIPE, not self-limiting, because of their extreme-developed lung diffusion. This could be aggravated when swimmers are exposed to strenuous swimming in cold water (4).