Background: In patients with COPD, the 6-minute walk test (6MWT) and the 30-second sit-to-stand test (30sec-STS) are widely used as clinical outcome measures of walking capacity, lower limb muscle strength, and functional ability. Due to a documented learning effect, at least two trials are recommended for assessment. The aim of our study was to investigate the intra- and inter-rater reliability and agreement of the two tests in patients with severe and very severe COPD (FEV1 <50%). Patients and methods: Fifty patients (22 females; mean [SD]: age 67 [9] years, FEV1 predicted 32 [9]%) were assessed with the 6MWT and the 30sec-STS twice by the same assessor on test-day 1 (T1) and by another assessor 7-10 days later on test-day 2 (T2). Results: The 6MWT intra- and inter-rater reliability (intraclass correlation coefficient, ICC1.1) was 0.98 (lower limit 95% CI: 0.94) and 0.96 (lower limit 95% CI: 0.94), respectively, and agreement (standard error of the measurement, SEM) was 14.8 and 20.5 m, respectively. The 30sec-STS intra- and inter-rater reliability and agreement results were, respectively, ICC1.1 0.94 (lower limit 95% CI: 0.90) and 0.92 (lower limit 95% CI: 0.86), with SEM of 0.97 and 1.14 repetitions. There was no difference (95% CI: -5.3; 8.1) between the 6MWT distances on T1, while the mean walking distance improved 7.9 m (0.0 m; 15.8 m) from T1 to T2. Improvement on the same test date was less likely (OR: 3.6 [95% CI: 1.1; 11.8], Fisher's exact test, P=0.047) in patients who walked less than 350 m in the 6MWT. We found no clinically relevant learning effect in the 30sec-STS. Conclusion: In patients with severe and very severe COPD the 6MWT and the 30sec-STS showed excellent intra- and inter-rater reliability and acceptable agreement. No learning effect was documented for the tests when performed on the same day. Our data suggest that in clinical practice using different assessors is acceptable, and that a single test trial may be sufficient to assess patients with severe and very severe COPD.