The reproducibility of 6-minute walking test (6MWT) needs to be more solidly studied. This study aimed to investigate the reproducibility of two 6MWT performed in subsequent days in a large and representative sample of patients with chronic obstructive pulmonary disease (COPD) and to quantify the learning effect between the two tests, as well as its determinants. In a retrospective observational study, 1514 patients with COPD performed two 6MWT in subsequent days. Other measurements included body composition (dual X-ray absorptiometry), dyspnoea (Medical Research Council scale) and comorbidity (Charlson index). Although the 6MWT was reproducible (ICC=0.93; p<0.0001), patients walked farther in the second test [391m (95%CI 155 to 585m) vs. 418m (185 to 605m); p<0.0001]. On average, the second 6MWT increased by 27m (or 7%), and 82% of patients improved in the second test. Determinants of improvement >/=42m in the second test (upper limit of the clinically important change) were: first 6MWT <350m, Charlson index <2 and BMI <30 kg.m(-2) (OR 2.49, 0.76 and 0.60, respectively). The 6MWT was statistically reproducible in a representative sample of patients with COPD. However, the vast majority of patients improved significantly in the second test by an average learning effect of 27m.