Bilateral idiopathic diaphragmatic paralysis (BIDP) may result in progressive ventilatory failure. To test the hypothesis that this is in part due to dysfunction of overtaxed inspiratory muscles, we studied 3 patients with BIDP before and after 2, 5, and 18 wk of daily intermittent external surface negative pressure ventilation (ENPV). The patients were evaluated using a zero to 10 functional score (FS) that graded dyspnea, orthopnea, capacity to perform activities of daily living, and ability to work. Pleural (Ppl), abdominal (Pab), and transdiaphragmatic (Pdi) pressures were used as an index of respiratory muscle function. All patients improved their functional score (FS increased 2, 6, and 6, respectively) and their pressure generating ability (Pplmax increased -18, -37, and -46 cm H2O, respectively). Forced vital capacity and functional residual capacity increased in the 2 patients ventilated for longer than 2 wk. These results indicate that ventilatory muscle dysfunction may result from chronic increased work of the inspiratory muscles and that it may improve after periods of intermittent ENPV. This may occur as early as 2 wk after initiation of therapy.