Background: In patients with chronic respiratory failure (CRF) nocturnal mechanical ventilation (NMV) confers increased exercise tolerance. The hypothesis tested in the present study was that the increased exercise performance is associated with increased quadriceps strength. Methods: In 28 patients with CRF due to chronic obstructive pulmonary disease and restrictive thoracic disease (post-tuberculosis-sequelae, scoliosis and obesity-hypoventilation) NMV was started. Before and after 2-month NMV the exercise tests, namely shuttle and 6-min walking distance, were performed. Furthermore, quadriceps strength was measured as the twitch tension elicited by magnetic stimulation the femoral nerve (TwQ) and the maximum voluntary contraction force (MVC). Results: After 2 months therapy with NMV there was significant clinical and blood gas improvement. NMV significantly improved the walking distance by approximately 18% but there was no improvement in TwQ or MVC, the data could exclude a 15% improvement in TwQ with 82% confidence. Conclusion: The strength of quadriceps does not change after 2 months of effective NMV in patients with CRF despite a marked increase in endurance time. Factors other than quadriceps strength account for the improved performance.