['Systemic effects of COPD include skeletal muscle dysfunction; the lactate threshold (LT) is an index of such dysfunction. However, it is not feasible in daily clinical practice to accurately determine the LT in all patients with COPD. There is no simple, practical and non-invasive index for determining the time at which rehabilitation should start. Previous studies have shown that the LT corresponds to the point at which the blood lactate concentration is 0.5 mmol/L above baseline (LTDelta0.5 mmol/L). The aim of the study was to assess the value of LTDelta0.5 mmol/L as an index of selecting candidates for pulmonary rehabilitation in COPD patients.', 'Eighteen male outpatients with COPD were enrolled. Exercise tolerance based on the 12-min walk test and treadmill exercise test, lactate concentrations and activities of daily living before and after individual exercise stress testing were assessed.', "There were no significant differences in the 12-min walk distance or metabolic equivalents between patients with and without a 0.5 mmol/L or more increase in lactate from baseline to post-walk test. However, significant differences were observed in activities of daily living, as assessed using Fletcher's scale, between those with and without a 0.5 mmol/L or greater increase in lactate from baseline to post-walk test."] Is arterial blood lactate a useful guide to when rehabilitation should be instigated in COPD?