We conducted a nested case-control analysis to further analyze the impact of COPD and various potential confounders on the risk of developing depression, suicidal ideation, or suicide. For this purpose we identified for each case with depression or suicidal ideation/suicide four control patients who were selected at random from the study population (ie, patients with or without COPD). Thus, controls did not develop depression, suicidal ideation, or suicide during follow-up. These controls were matched to cases on age, sex, practice, and index date (ie, the date when the case had the incident diagnosis of depression, suicidal ideation, or committed suicide). We then compared the prevalence of COPD between case patients and their controls using conditional logistic regression analyses. We also stratified patients with COPD by COPD severity. As a surrogate marker for COPD severity we used COPD treatment, similar to a previous approach published by Soriano et al.17 We categorized patients with COPD into mild COPD (patients who received no drug treatment), moderate COPD (patients who received at least one prescription for short-acting anticholinergics, β-agonists, tiotropium, leukotriene receptor antagonists, inhaled steroids, or xanthines), or severe COPD (patients who needed oxygen treatment). We adjusted the case-control analyses for patient characteristics such as BMI ( < 17.5, 17.5-24.9, 25-29.9, 30+ kg/m2, or unknown), smoking history (no, current, past, unknown), as well as for various comorbidities and drugs.