AIM: The aim of this investigation was (1) to study the effect of untreated chronic heart failure (CHF) on alveolar membrane diffusion capacity (transfer coefficient, K(CO)) in elderly patients and (2) to study the impact of the standard regime of medical treatment with diuretics and ACE-inhibitor/angiotensin-II receptor antagonists on K(CO) in these patients. METHODS: Non-medicated patients (except for diuretics) with symptoms of heart failure (NYHA II-III) and echocardiographically estimated left ventricular ejection fraction (LVEF) <0.40 were recruited. All were characterized according to the results of multiple ECG-gated radionuclide ventriculography (MUGA). LVEF<0.50 when measured by MUGA was considered as heart failure (HF). A total of 20 patients fulfilled the criteria. All patients had a lung function test including measurement of K(CO) and a MUGA for LVEF measurement performed prior to medical treatment (baseline) and after 1 year of treatment with diuretics and ACE-inhibitors/angiotensin-II receptor antagonists. Age- and gender-matched healthy volunteers were included as control group. RESULTS: (mean+/-S.E.M.): K(CO) at baseline was 0.95+/-0.06 and 1.25+/-0.04 mmol/min x kPa/l in HF patients and controls, respectively (p<0.05). After 1 year of treatment, K(CO) was normalized in the HF group (1.23+/-0.13 mmol/s x kPa, p<0.05). LVEF increased in the HF group from 0.28+/-0.03 at baseline to 0.34+/-0.03 after 1 year of treatment (p<0.05). CONCLUSION: Elderly patients with symptomatic HF (NYHA II-III) and reduced systolic function have respiratory dysfunction in the form of reduced K(CO). One year of medical treatment had a significant beneficial effect on K(CO) and LVEF.