Exercise challenge test (ECT) may help in the diagnosis of asthma. Asthma is season dependent (relative summer nadir).This study was designed to prospectively show that ECT sensitivity decreases when performed in summer and/or out of personal asthma season. We performed two ECTs in 49 patients with suspected asthma, at baseline and at least 6 months later. Test positivity was defined as a ≥10% drop of forced expiratory volume in 1 second after exercise. Patients were defined as having exercise-induced bronchoconstriction (EIB) if at least one of the two tests was positive. We analyzed the impact of season and of patient-reported asthma season on ECT sensitivity. We performed 21 ECTs in the summer and 77 in other seasons. Summer tests were positive in 14.3% of patients versus 33.8% in other seasons (p = 0.11). In patients with confirmed EIB, the sensitivity (95% CI) of a summer test was 0.25 (0.007-0.57), compared with 0.76 (0.58-0.89) in other seasons. Tests performed out of personal asthma season were positive in 10.0% of tests compared with 37.1% of tests during personal asthma season (p = 0.007). In patients with confirmed EIB, the sensitivity (95% CI) of a test performed out of asthma season was 0.27 (0.07-0.61), compared with 0.74 (0.55-0.87) during season. The sensitivity of ECT performed in the summer or outside of the personal asthma season is about one-third of ECT performed not in the summer or during personal asthma season. We suggest performing ECTs during personal asthma season and not in the summer if there is no asthma season known.