Background: The clinical features of idiopathic pulmonary fibrosis (IPF) and chronic hypersensitivity pneumonitis can be indistinguishable; the need to eliminate occult environmental factors known to cause pulmonary fibrosis in patients suspected to have IPF during diagnostic evaluation is evident. We aimed to investigate occult, putative causes in the environments of patients diagnosed with IPF using tests beyond those conventionally used. Methods: In this case-cohort study, 60 consecutive patients diagnosed with IPF on the basis of the 2000 American Thoracic Society (ATS) and the European Respiratory Society (ERS) criteria were prospectively followed up every 4 months for 6 years between Jan 1, 2004, and Dec 31, 2009. At each visit a uniformly applied questionnaire was administered to these 60 patients to identify occult antigen exposure known to cause hypersensitivity pneumonitis. Patients underwent specific IgG determination, bronchoalveolar lavage, bronchial challenge testing with suspected antigens, and re-review of histopathological features in existing and subsequently obtained surgical lung biopsy samples and from lung explants. Specimens obtained from suspected sources from the patient's environment were subjected to cultures in microbiology laboratory. These clinical data and discussions among pulmonologists and radiologists familiar with IPF were used to confirm the diagnosis in accordance with 2011 ATS, ERS, Japanese Respiratory Society, and Latin American Thoracic Association guidelines; 46 of the 60 patients had IPF according to the 2011 guidelines, and our analyses in this study were focused on these 46 patients. Findings: 20 of the 46 (43%, 95% CI 29-58) patients with IPF according to 2011 guidelines had a subsequent diagnosis of chronic hypersensitivity pneumonitis: nine patients had positive bronchial challenge testing (eight of whom were also IgG positive and six of these patients also had surgical lung biopsy showing a pattern consistent with chronic hypersensitivity pneumonitis); seven were IgG positive plus had histopathology on surgical lung biopsy that was consistent with hypersensitivity pneumonitis; one was IgG positive plus had greater than 20% lymphocytes in bronchoalveolar lavage fluid; and three had findings on surgical lung biopsy that were consistent with subacute hypersensitivity pneumonitis (and IgG positive). Altogether, 29 of 46 patients diagnosed with IPF who had met the 2011 criteria had lung tissue available for histopathology (surgical lung biopsy in 28 patients and explanted lung in two patients, one of whom also had surgical biopsy) during the study period, and 16 of the 20 patients with chronic hypersensitivity pneumonitis had histopathological features on surgical lung biopsy that were consistent with this diagnosis. 26 of the 46 patients remained with a diagnosis of IPF. Interpretation: Almost half of patients diagnosed with IPF on the basis of 2011 criteria were subsequently diagnosed with chronic hypersensitivity pneumonitis, and most of these cases were attributed to exposure of occult avian antigens from commonly used feather bedding. Our results reflect findings in one centre with recognised expertise in chronic hypersensitivity pneumonitis, and further research and studies at other centres are warranted. Funding: Fondo de Investigaciones Sanitarias; Fundació Privada Cellex; SEPAR 2010. © 2013 Elsevier Ltd.