We studied 12 asymptomatic conscripts affected by bronchial asthma to evaluate the level of hypoxemia and its relationship with bronchial obstruction induced by methacholine (Mch) challenge. Arterial blood samples were obtained before and immediately after bronchial challenge. The range of decrease of forced expiratory volume/forced vital capacity x 100 in 1 second of expiration (FEV1), the absolute value reported as a percentage of the baseline value, after Mch challenge was from 21% to 36%. The mean decrease of arterial oxygen tension was 27 mm Hg (from 11.4 to 39.8 mm Hg), the mean decrease of hemoglobin saturation was 3.53% (from 0.4 to 7.2), and the mean increase of alveolar-arterial gradient for oxygen was 28.2 mm Hg. We found a significant correlation between the prechallenge and postchallenge differences of arterial oxygen tension and FEV1 (DFEV1P), which was expressed as a percentage of the predicted value. In conclusion, the Mch challenge seems to be a safe test, and the decrease of arterial oxygen tension can be sufficiently predicted by DFEV1P.