Intensity of perceived inspiratory difficulty was investigated in 17 patients with severe respiratory insufficiency due to muscle disease, compared with healthy matched controls. Subjects breathed through a threshold valve generating a constant inspiratory negative pressure proportional to their maximal inspiratory pressure. Four load levels ranging from 10 to 40% of the maximal inspiratory pressure were applied in random order. Patients had significantly less perceived inspiratory difficulty than controls at each load level expressed as a percentage of maximal inspiratory pressure P < 0.001. However, when the load was expressed as the absolute value, the slope of the Borg scale score versus mouth pressure was similar in the two groups (P = 0.11). The ventilatory pattern remained unchanged in each group as the load increased. We conclude that in patients with myopathy, loads leading to respiratory muscle fatigue (40% of maximal inspiratory pressure) may fail to produce perceived inspiratory difficulty.