In fibrotic lung diseases, there is increasing evidence that the extent of lung fibrosis on CT is an important predictor of prognosis. For example, in a multicenter study1– of patients with idiopathic pulmonary fibrosis enrolled in a treatment trial, the CT extent of lung disease, evaluated visually using a semiquantitative scoring system, was the strongest independent predictor of survival (p < 0.0001). Similarly, in the control arm of the recently published Scleroderma Lung Study,2– the CT extent of lung fibrosis at baseline was a strong predictor of FVC at 12 months (adjusted for baseline FVC) [p = 0.006]. In both of these studies, a semiquantitative visual scoring system was used to evaluate the extent of disease on CT. However, semiquantitative scoring systems are limited by the requirement for expert radiologists, and by moderate interobserver variation.3–5 CT has been used to accurately quantify emphysema for almost 20 years,6–7 but the development of quantitative CT-based measures for lung fibrosis has been more challenging. In the current exciting era of clinical trials for idiopathic pulmonary fibrosis and other fibrotic lung diseases, quantitative CT would potentially offer an objective, reproducible measure of disease extent in these conditions.