Chronic obstructive pulmonary disease (COPD) is a multisystemic inflammatory disease characterized by pulmonary and extrapulmonary symptoms. The impaired lung function has long-term implications on metabolism and homeostasis of many organ systems such as the skeleton, heart, brain and skeletal muscle. The occurrence and prevalence of anemia in COPD has rarely been studied. Anemia is such a common and simple clinical finding that we may underestimate its physiological relevance in COPD. The aim of the study was to retrospectively investigate the prevalence of anemia in a large population of COPD patients and to compare it to patients with chronic heart failure, renal insufficiency, cancer and asthma. A population of 7337 patients that was treated in the University Hospital Charité, Berlin, Germany, from 1996 to 2003 was subsetted according to the ICD-9/10 code of the discharge diagnoses into the above-mentioned diagnoses groups. The overall prevalence of anemia in COPD patients was 23.1%. It was comparable to the prevalence of anemia we found in patients with chronic heart failure (23.3%). Patients with renal insufficiency and cancer presented the highest anemia frequencies. The high prevalence of anemia in hospitalised COPD patients that were treated mostly for exacerbations gives evidence that anemia is also a comorbidity in COPD and may contribute to exercise limitation and dyspnoea.