To the Editor:

“High risk ” groups for exacerbations of chronic obstructive pulmonary disease (COPD) in the 2011 Global Initiative for Chronic Obstructive Lung Disease (GOLD) proposal (i.e. groups C and D) [1] include: patients with a forced expiratory volume in 1 s FEV1 <50 % reference and <2 exacerbations year-1 (subgroups C1 and D1); patients with ⩾2 exacerbations·year-1 and an FEV1 ⩾50 % reference (subgroups C2 and D2); and patients with both FEV1 <50 % ref. and ⩾2 exacerbations·year-1 (subgroups C3 and D3) [2–5]. We hypothesised that these high-risk subgroups will differ in other clinical, functional and biological characteristics and will be associated with different long-term outcomes. We explored this hypothesis in the ECLIPSE (Evaluation of COPD Longitudinally to Identify Predictive Surrogate Endpoints) cohort [6, 7]. The design and methodology of the ECLIPSE study (www.clinicaltrials.gov with identifier number NCT00292552; GSK study code SCO104960) has been published elsewhere in detail [6]. The study was approved by the Ethics Committees from the participating centres and all participants signed their informed consent [7]. Out of the 2164 GOLD grades II-IV patients included in the ECLIPSE study, 2101 (97%) had complete GOLD 2011 data [4] and were included in the current analysis; 1313 of them (62.5%) were classified as groups C (n=483, 36.8%) or D (n=830, 63.2%), using the modified Medical Research Council dyspnoea score to determine high and low COPD symptoms