Reliability of the maximal inspiratory pressure (Pimax) was examined by measuring Pimax once a week for 4 weeks in 91 patients with chronic obstructive pulmonary disease using an aneroid pressure gauge. Five Pimax trials were conducted at each test. From the first to the fourth test, the Pimax increased by a mean of 9 cm H2O (SD = 10). From the third to the fourth test, Pimax increased by a mean of 2 cm H2O and performance appeared to be plateauing. The test-retest reliability coefficient was r = 0.97 for Pimax measured at the third and fourth test session. The 95 percent confidence interval for the absolute difference in Pimax at the third and fourth test was 3 to 5 cm H2O. We conclude that performance of Pimax improves with practice in naive COPD patients and Pimax is reliable when measured with an aneroid gauge by experienced data collectors if patients are given sufficient practice.