This case study aimed to report blood markers and resting respiratory rate (RR) oscillations at sea level, during a 5-wk 3900 m altitude sojourn, and after returning to sea level in a 36-year-old professional wheelchair marathoner. Outcome measures plasma erythropoietin (EPO) concentration, hemoglobin, reticulocytes count, erythrocytes and hematocrit as well as RR were measured upon wakening 7-weeks pre-altitude, 7-days pre-altitude, 35 hours after arrival to altitude, on days 8, 15, 21, 28 and 35 at altitude, 6 and 16 days after returning to sea level. EPO increased up to 259 % (31.6 U l-1) 35 hours upon arrival at altitude and decreased below pre-altitude level (12.2 U l-1) on the 21st day of the camp (8.7 U l-1), reaching the lowest values 16 days after returning from altitude (1.9 U l-1). All blood parameters, except for reticulocytes, increased (range: +17.9 to +23.8%) after 35 days of altitude exposure. Compared to pre-altitude, RR increased during the first week of exposure to hypoxic conditions and remained elevated throughout the camp until the fifth week (5.1±0.4 vs. 9.1±1.6 and 6.6±0.8 breaths min-1; Cohen´s d = +3.4 and +2.4, respectively). A 5-wk high-altitude sojourn triggered polycythemia and elevations in RR (as indicators of effective hypoxic acclimatization) in a professional wheelchair-marathoner.