OBJECTIVE: Circulating fibrocytes are elevated in idiopathic pulmonary fibrosis, but the relationship between fibrocyte level with lung function decline and outcomes is lacking replication in prospective clinical study. We aim to validate the utility of circulating fibrocyte levels as a prognostic biomarker in idiopathic pulmonary fibrosis. METHODS: We tested associations between circulating fibrocyte levels, mortality, disease progression and longitudinal lung function in a well-defined prospective observational study of pulmonary fibrosis (PROFILE; NCT01134822). A subset of recruited participants had blood samples processed for fibrocyte measurement, with flow cytometry based on CD45 and collagen-I gating. Associations were tested using univariable and multivariable generalised linear models. Mortality data were subsequently combined with an independent cohort in a mixed-effect multilevel analysis. RESULTS: In 102 participants with idiopathic pulmonary fibrosis, a previously defined mortality risk threshold of 5% circulating fibrocytes was not reproducible. An empirically defined cutpoint of 2.22% was associated with a greater risk of overall mortality in adjusted analysis (Hazard Ratio 2.24 95% CI 1.06-4.72). A 2.5 fold greater risk of mortality was supported in a pooled analysis with a historic cohort for a larger sample of 162 participants with idiopathic pulmonary fibrosis (Hazard Ratio 2.49 95% CI 2.41-2.56). We found no association of fibrocytes with lung function or disease progression. CONCLUSIONS: In a large sample of circulating fibrocytes from people with idiopathic pulmonary fibrosis, levels of 2.22% or above were associated with greater mortality, but not with disease related decline in lung function.