Change in body composition with skeletal muscle wasting, a major component of pulmonary cachexia, is associated with mortality in chronic obstructive pulmonary disease and cancer. However, its relevance in interstitial lung diseases (ILDs) remains unclear. We hypothesized changes in body composition would be associated with mortality in ILDs. We measured the cross-sectional-area (ESM_CSA) and muscle attenuation (ESM_MA) of erector-spinae muscles, as determined by CT-imaging, in patients with idiopathic pulmonary fibrosis (IPF; n = 131) and idiopathic pleuroparenchymal fibroelastosis (iPPFE; n = 43) and controls. Subsequently, implications with prognosis were evaluated. The ESM_CSA of ILD patients, but not ESM_MA, was significantly smaller than that in controls. Lower ESM_CSA with decreased BMI were recorded in iPPFE patients versus IPF patients, whilst IPF patients had decreased ESM_CSA without BMI decline. Lower ESM_CSA in IPF patients were associated with poorer prognoses. Conversely, decreased ESM_MA were associated with worse survival in iPPFE patients. Multivariate analyses showed that ESM_CSA in IPF and ESM_MA in iPPFE were independent risk factors for mortality. Distinct changes in body composition had prognostic significance among patients with IPF and iPPFE. Lower ESM_CSA and ESM_MA were independently associated with poor prognosis in IPF and iPPFE, respectively. These results suggest values to measure body composition changes in managing patients with IPF and iPPFE.