Background: In clinical practice, patients have different inspiratory behaviors during noninvasive pressure support ventilation (PSV): some breathe quietly, others actively help PSV by an additional effort, and others even resist the inspiratory pressure of PSV. Objective: What is the influence of patient collaboration (inspiratory behavior) on the efficiency of PSV? Methods: We ventilated 10 normal subjects with nasal PSV (inspiratory/expiratory: 10/0 and 15/5 cm H₂O) and measured their flow and volume with a pneumotachograph and their esophageal and gastric pressures during three different respiratory voluntary behaviors: relaxed inspiration, active inspiratory work and resisted inspiration. Results: When compared with relaxed inspiration with 10/0 cm H₂O PSV: (1) an active inspiratory effort increased tidal volume (from 789 ± 356 to 1,046 ± 586 ml; p = 0.006), minute ventilation (from 10.40 ± 4.45 to 15.77 ± 7.69 liters/min; p < 0.001), transdiaphragmatic work per cycle (from 0.55 ± 0.33 to 1.72 ± 1.40 J/cycle; p = 0.002) and inspiratory work per cycle (from 0.14 ± 0.20 to 1.26 ± 1.01 J/cycle; p = 0.003); intrinsic positive end-expiratory pressure (PEEP_i) increased from 1.23 ± 1.02 to 3.17 ± 2.30 cm H₂O; p = 0.002); (2) a resisted inspiration decreased tidal volume (to 457 ± 230 ml; p = 0.007), minute ventilation (to 6.93 ± 3.04 liters/min; p = 0.028) along with a decrease in transdiaphragmatic work but no change in PEEP_i. Data obtained during a bilevel PSV of 15/5 cm H₂O were similar to those obtained with the 10/0 cm H₂O settings. Conclusions: Active inspiratory effort increases ventilation during PSV at the expense of an increased breathing work and PEEP_i. Resisted inspiration inversely decreases inspiratory work and ventilation with no air trapping. These differences between inspiratory behaviors could affect the expected beneficial effects of PSV in acutely ill patients.