Continuous positive pressure (CPAP) is the main treatment of the obstructive sleep apnoea syndrome (OSAS). We assessed its effects on the upper airway (UA) dynamics in response to bilateral anterior magnetic phrenic nerve stimulation (BAMPS) in 17 awake untreated OSAS patients (15 males; 52 ± 7 years) whose effective CPAP (P_eff) had been determined beforehand by a conventional titration sleep study. All twitch-related inspirations were flow-limited, flow first rising to a maximum ( V_Imax), then decreasing to a minimum ( V_Imin), and then increasing again (M-shaped pattern). Up to V_Imin, the relationship between driving pressure (P_d) and flow ( V) could adequately be fitted to a polynomial regression model ( V = k₁P_d + k₂P_d²; r² = 0.71-0.98, P < 0.0001). At atmospheric pressure V_Imax was 700 ± 377 ml s^-1, V_Imin was 458 ± 306 ml s^-1, k₁ was 154.5 ± 63.9 ml s^-1 (cmH₂O)^-1, and k₂ was 10.7 ± 7.3 ml s^-1 (cmH₂O)^-1. CPAP significantly increased V_Imax and V_Imin (peak values 1007 ± 332 ml and 837 ± 264 ml, respectively) as well as k₁ and k₂ (peak values 300.9 ± 178.2 ml s^-1 (cmH₂O)^-1 and 55.2 ± 65.3 ml s^-1 (cmH₂O)^-1, respectively). With increasing CPAP, k₁/k₂ increased up to a peak value before decreasing. We defined as P_eff,stim the CPAP value corresponding to the highest k₁/k₂ value. P_eff,stim was correlated with P_eff (P_eff = 7.0 ± 2.0; P_eff,stim = 6.4 ± 2.6 cmH₂O; R = 0.886, 95 % CI 0.696-0.960, P < 0.001). We conclude that CPAP improves UA dynamics in OSAS and that the therapeutic CPAP to apply can be predicted during wakefulness using BAMPS.