Patients with chronic renal failure (CRF) and haemodialysis treatment usually have a reduced CO transfer factor. The aim of this study was to evaluate the effects of alveolar wall fibrosis and of anaemia on gas diffusion in the lungs. The NO and CO transfer factors of the lung (TLNO and TLCO) were measured, simultaneously, in 15 patients haemodialysed three times a week for 1-10 yrs. Assuming that NO is highly reactive with blood, Tlno is thus directly proportional to the membrane diffusion factor (DmCO). The lung capillary blood volume (Vc) was derived from the set of the two transfer equations. Transfer factors were measured between haemodialysis sessions. All patients but one were anaemic, with haemoglobin concentrations ranging 61-151 g.l-1. All had decreased Vc, and a decreased DmCO was observed in 14 patients. However, after correction for the anaemia, Vc values were normal with the exception of three patients. The percentage decrease in DmCO with respect to normal was correlated with the time elapsed since the first haemodialysis. These results support the idea of a progressive development of haemodialysis-induced chronic lung disease, that may be related to a mechanism of complement activation by a bio-incompatible membrane (Cuprophane). Accordingly, patients with compromised cardiopulmonary functions should be dialysed with a bio-compatible membrane.