INTRODUCTION Acute exposure to moderate altitude (2.250m) increases the diffusion capacity for carbon monoxide (DLCO) (1) but after a 3 weeks altitude camp exposition (2.250m) there is a decrease in the DLCO (2) values. Swimming practice induces a marked increase in lung capacity and lung diffusion (3,4) over the training development, and elite swimmers use to train at stages located at moderate altitude. The aim of this study is to evaluate the changes in DLCO after 14 days of altitude camp in elite swimmers and the acute effect of an altitude combined session of swimming training at hypobaric altitude of 1.850m + aerobic cycling session at normobaric simulated altitude of 3.000m METHOD The participants are 7 international level swimmers, 3 men and 4 women, from 17 to 24 years old, with more than 3 years of experience in altitude training. The training schedule consists of 25-30 hours of training per week. The swimming session lasts 150 minutes and the distance covered is 7.500m at moderate intensity and the cycling session consists of 50 minutes at low intensity. The material used is a computerized spirometer with single-breath method to measure the changes in DLCO (5) (Easy One Pro, ndd, Zurich Switzerland). RESULTS There are no changes in DLCO after 14 days of altitude training camp in our sample (42,0±11,9 vs. 40,2±12,7). Likewise there are no changes after a swimming training at 1.850m + an aerobic cycling session at 3.000m in a normobaric hypoxic chamber (45,8±14,4 vs. 45,2±11,8 vs. 41,1±12,6). DISCUSSION Altitude exposure and exercise increase the mechanical stress on the pulmonary system, leading to subtle changes in the permeability of the lungs which, normally, may not be an inconvenience in the extreme-developed lungs from elite athletes. This new study shows that elite swimmers with experience in altitude training have not changes in DLCO during and at the end of 14 days altitude of international level training camp. Despite of there are no acute changes after a combined session of swimming (1.850m) and cycling (3.000m), a slight decrease is appreciated after cycling. Further research including more participants and with a higher heterogeneity in the bronquial ability (asthma) are required to assess this tendency.