OBJECTIVE To assess the discontinuous incremental threshold loading (DC-ITL) test as a measure of respiratory muscle endurance for patients with COPD in terms of perceived breathing difficulty, reliability, and validity. METHODS The DC-ITL test was repeated three times at weekly intervals under identical test conditions. METHODS Clinical research laboratory. METHODS Forty-eight patients with moderate to severe COPD. RESULTS Rating of perceived breathing difficulty (RPBD) was measured at the end of each stage of the DC-ITL test with a Borg category-ratio scale. The maximal inspiratory pressure (PImax) was measured before and after the DC-ITL test. Breathing patterns were measured during the DC-ITL test. The mean (+/-SD) for RPBD at the maximal load was 6.3 (3.1), 6.6 (2.8), and 6.7 (2.7) for visits one, two, and three, respectively (not significant). The mean relative maximal load for the DC-ITL test (peak mouth pressure as a percent of PImax) at the last completed stage was 59+/-23%, 62+/-20%, and 63+/-19% for visits one, two, and three, respectively (not significant). Test-retest reliability was r1,2=0.82 and r2,3=0.69 for relative maximal load and r1,2=0.90 and r2,3=0.90 for absolute maximal load (peak mouth pressure). Tidal volume decreased (p < 0.01) and respiratory rate increased (p < 0.01) from the next-to-the-last to the last completed stage. PImax decreased after the DC-ITL test (p < 0.01). CONCLUSIONS Moderate breathing difficulty was experienced during the DC-ITL test. The test was reliable and the results of this study support its validity as a measure of respiratory muscle endurance.