The RDW is a measurement of the variability in the size of circulating RBCs and is commonly reported as part of a CBC. Higher values indicate greater variability in cell size, whereas lower values indicate more homogeneity. Although small amounts of variation in cell size are normal, significant variation has been linked to worse prognoses in several diseases, particularly cardiovascular disorders.16‐20 However, the pathophysiologic basis for the association between the RDW and IPF mortality is currently unknown. Elevated RDW values may be caused by an underlying state of inflammation that induces changes in erythropoiesis, RBC circulation half-life, and RBC membrane deformability. The higher mean pulmonary artery pressure in the high RDW group raises the possibility that the RDW value is a surrogate for underlying pulmonary hypertension, which could explain the higher mortality in this group. Indeed, the RDW has been shown to be a powerful predictor of outcomes in patients with idiopathic pulmonary arterial hypertension.24 It is also possible that the RDW value might be a marker for other comorbidities, such as coronary artery disease, which may impact survival.25,26 RDW value has been linked to age, nutritional status, smoking, and lung function.27,28 However, none of these are likely to explain our findings; specifically, age and lung function were included in our multivariate analysis, and there was no difference in smoking history between the groups. Also, IPF is generally a condition that is not accompanied by severe malnutrition, and indeed the BMI, albeit a poor correlate, was similar between groups. It is also possible that the vascular distortion that accompanies the fibrotic process may cause a low-grade hemolysis, which results in a compensatory increase in erythropoiesis. The lower HgB levels noted in the high RDW group could also be due to low-grade hemolysis. These mechanistic hypotheses are highly speculative and require further investigation in follow-up studies.