The single breath DLCO (TLCO) has proved as an essential part of the routine pulmonary function screen, similar to spirometry. In spite of nearly 100 years research, there is still concern over the relative importance of the alveolarcapillary membranes versus the red cells as rate limiting steps in the overall transfer of carbon monoxide from gas to blood, but this is only a quantitative problem. The essential nature of the DLCO has already been elucidated by F.J.W. Roughton and R.E. Forster having played the major roles. Interpreting the DLCO, in conjunction with spirometry and lung volumes assessment, may assist in diagnosing the underlying disease and in the Rheumatology field it is essential it's knowledge because it offers the possibility of establish the differential diagnosis and a close follow-up of the patients with pulmonary manifestations in autoimmune diseases.