Background: The National Heart, Lung, and Blood Institute-sponsored IPF Clinical Research Network (IPFnet) studies enrolled subjects with idiopathic pulmonary fibrosis (IPF) to evaluate drug therapies in treatment trials. An adjudication committee (AC) provided a structured review of cases in which there was uncertainty or disagreement regarding diagnosis or clinical event classification. This article describes the diagnosis and adjudication processes. Methods: The diagnostic process was based on review of clinical data and high-resolution CT scans with central review of lung biopsies when available. The AC worked closely with the data coordinating center to obtain clinical, radiologic, and histologic data and to communicate with the clinical centers. The AC used a multidisciplinary discussion model with four clinicians, one radiologist, and one pathologist to adjudicate diagnosis and outcome measures. Results: The IPFnet trials screened 1,015 subjects; of these, 23 cases required review by the AC to establish eligibility. The most common diagnosis for exclusion was suspected chronic hypersensitivity pneumonitis. The AC reviewed 88 suspected acute exacerbations (AExs), 93 nonelective hospitalizations, and 16 cases of bleeding. Determination of AEx presented practical challenges to adjudicators, as necessary clinical data were oft en not collected, particularly when subjects were evaluated outside of the primary study site. Conclusions: The IPFnet diagnostic process was generally efficient, but a multidisciplinary adjudication committee was critical to assure correct phenotype for study enrollment. The AC was key in adjudicating all adverse outcomes in two IPFnet studies terminated early because of safety issues. Future clinical trials in IPF should consider logistical and cost issues as theyincorporate AExs and hospitalizations as outcome measures.