Objective: The present study investigated the acute effects of a mixed-modality, long-duration adventure race on pulmonary function in adolescent athletes. Methods: Twenty male adolescents aged 14 to 17 years volunteered to participate in a simulated competitive wilderness adventure race of 68.5-km. Expiratory function was evaluated by spirometry with an Ergocard CPX Clinical system before, immediately after, and 24 h after race completion. Measurements included forced vital capacity (FVC), forced expiratory volume in 1 s (FEV1), forced expiratory flows at 25%, 50% and 75% of FVC (FEF25, FEF50, FEF75, respectively) and peak expiratory flow (PEF). Maximal inspiratory and expiratory mouth static pressures (MIP and MEP, respectively) were also measured using a portable hand-held mouth pressure meter across the same time points. Results: The mean completion time of the race was 05:38 ± 00:20 hours. A significant post-race decrease in FVC was observed immediately after the race (-5.2%, p < 0.05). However, no significant changes were observed for FEV1, PEF or the FEV1/FVC and FEV1/PEF ratios. The results also showed no significant modifications in the maximal expiratory flow-volume curves irrespectively of FVC percentages (FEF25, FEF50, FEF75). In addition, estimates of respiratory muscle strength (MIP and MEP) were unaffected by the race. Conclusion: The long-duration adventure race induced no substantial reduction in expiratory pulmonary function and this response was associated with no apparent respiratory muscle fatigue. Therefore, the pulmonary system of trained adolescent athletes was sufficiently robust to endure the mixed-modality, long-duration adventure race of ∼5-6 h.