Rationale: Heterozygous mutations in the coding regions of the telomerase genes, TERT and TERC, have been found in familial and sporadic cases of idiopathic interstitial pneumonia. All affected patients with mutations have short telomeres. Objectives: To test whether telomere shortening is a frequent mechanism underlying pulmonary fibrosis, we have characterized telomere lengths in subjects with familial or sporadic disease who do not have coding mutations in TERT or TERC. Methods: Using a modified Southern blot assay, the telomerase restriction fragment length (TRFL) method, and a quantitative-PCR assay we have measured telomere lengths of genomic DNA isolated from circulating leukocytes from normal controls and subjects with pulmonary fibrosis. Main Results: All affected patients with telomerase mutations, including cases heterozygous for newly reported mutations in TERT, have short telomere lengths. A significantly higher proportion of probands with familial pulmonary fibrosis (24%) and sporadic cases (23%) in which no coding mutation in TERT or TERC was found had telomere lengths <10^th percentile when compared to controls (P-value = 2.6 x 10^-8). Pulmonary fibrosis affectation status was significantly associated with TRFLs, even after controlling for age, sex and ethnicity (P-value = 6.1 x 10^-11). Overall, 25% of sporadic cases and 37% of familial cases of pulmonary fibrosis had telomere lengths <10^th percentile. Conclusions: A significant fraction of individuals with pulmonary fibrosis have short telomere lengths that cannot be explained by coding mutations in telomerase. Telomere shortening of circulating leukocytes may be a marker for an increased predisposition toward the development of this age-associated disease.