BACKGROUND: The aim of this work was to investigate the short-and long-term test-retest reliability of the 6-min walk distance (6MWD), peak heart rate, and nadir oxygen desaturation in idiopathic pulmonary fibrosis (IPF). METHODS: A reliability study of 70 adults with IPF was undertaken within out-patient pulmonary rehabilitation programs at 2 tertiary hospitals. Participants completed 2 baseline 6-min walk tests using a standard protocol, with continuous measures of percutaneous SpO2 and heart rate via pulse oximetry. The 6-min walk test was completed immediately following an intervention period and 6 months after. Reproducibility was assessed by intraclass correlation coefficient and Bland-Altman analysis. RESULTS: Participants with a mean = SD diffusing capacity of the lung for carbon monoxide of 48 = 14% were included. The reliability of the 6MWD was high (intraclass correlation coefficient = 0.96) with a mean learning effect of 21 m (95% CI 12–30 m). The learning effect persisted at 8 weeks (mean 14 m, 95% CI 5–23 m) but not 6 months (mean 15 m, 95% CI-1 to 30 m). Using the best (greatest) 6MWD significantly reduced the proportion of participants who were classified as having a clinically important response to rehabilitation compared with using the first 6MWD (40% vs 54%, P = .002). Nadir SpO2 was reproducible, with a mean difference of 0.7 = 2.2%, and limits of agreement of-4 to 5%. Peak heart rate was more variable, with mean difference 5 = 9 beats/min and limits of agreement of-12 to 20 beats/min. CONCLUSIONS: The 6MWD is a reproducible measure of exercise capacity in people with IPF. Whereas the nadir SpO2 may be accurately determined from one test, evaluating change in 6MWD with interventions may require 2 tests on each occasion. (ClinicalTrials.gov registration NCT0016828.) Key words: walk test; pulmonary fibrosis; reliability. [Respir Care 2018;63(8):994 –1001.