OBJECTIVE. The aim of our study was to retrospectively determine predictive factors for a decline in forced vital capacity (FVC) on initial CT using texture-based automated quantification in patients with idiopathic pulmonary fibrosis (IPF). MATERIALS AND METHODS. For our study, 193 patients with IPF and 1-year follow-up pulmonary function tests were enrolled in our study. A texture-based automated system used in-house software to quantify six regional CT patterns: normal, ground-glass opacity (GGO), reticular opacity (RO), honeycombing, emphysema, and consolidation. A decline of FVC was defined as a decrease in the initial FVC of more than 10%. RESULTS. A decline of FVC occurred in 32 patients: The mean volume of the decline in FVC was 0.43 +/- 0.18 (SD) L. The mean extents of GGO, RO, honeycombing, emphysema, and consolidation in all 193 patients were as follows: 12.3% +/- 11.9%, 16.8% +/- 9.8%, 7.1% +/- 6.7%, 3.9% +/- 5.5%, and 2.8% +/- 0.8%, respectively. A multivariate analysis revealed that RO was the sole independent predictor for a decline in FVC (p = 0.012; adjusted odds ratio, 1.047). ROC analysis showed that the AUC of RO was 0.641 and that the optimal RO cutoff value was 22.05% (sensitivity, 50.0%; specificity, 81.4%; negative predictive value, 89.1%). CONCLUSION. RO of less than 22.05% in extent can accurately predict stable IPF at 1-year follow-up in terms of FVC.