Using a "single breath" method, diffusing lung capacity (DLCO) and unit DLCO (KCO) were measured in 19 patients with chronic sideropenic anemia and 19 healthy women who were without any clinical or radiologic evidence suggestive of cardiorespiratory disease. Anemic patients, compared to the control group, had significantly reduced DLCO (% predicted) and KCO (% predicted) (p < 0.01). Each 10 g/L hemoglobin (Hb) reduction below a normal value of 136 g/L corresponded to 2% DLCO% and 3% KCO% reductions. For all examinees, where the range of Hb was from 50 to 150 g/L, a significant positive low linear correlation was noted between DLCO% and HB (r = 0.31; p = 0.049) and KCO% and Hb (r = 0.43; p = 0.005). Analyzing only anemic patients, where the range of Hb was from 50 to 109 g/L, no significant correlation was found between Hb and DLCO% or KCO%. Therefore, in patients with chronic sideropenic anemia DLCO is lowered less then expected, so one should not adjust measured DLCO or KCO values to standard Hb concentration, because probably various pathophysiological compensatory mechanisms affect DLCO components in unknown manner, changing the relationship between Hb and DLCO.