The histologic manifestation of idiopathic pulmonary fibrosis (IPF) is usual interstitial pneumonia (UIP), which is a good prognostic determinant of survival compared with other histologic interstitial lung disease patterns. According to the current international guidelines, the histologic features of suspected IPF/UIP are divided into 4 categories: UIP, probable UIP, possible UIP, and not UIP pattern. Four pulmonary pathologists who were blinded to clinicoradiologic information reevaluated 50 surgical lung biopsies (83.3%), 6 lung explant (10.0%), and 4 autopsy samples (6.7%) from the FinnishIPF registry (N=60) using the current diagnostic guidelines. Additional histologic features atypical for UIP were also evaluated and compared with clinicora-diologic information. The interobserver agreement of pathologists was examined by Cohen kappa (κ) coefficient; the survival of the patients was estimated with Kaplan-Meier curves. The histologic reevaluation indicated that 38 of 60 patients (63.3%) had definite UIP. Inflammation was the most common additional histologic finding (15/60, 25.0%). The interobserver agreement on histologic diagnosis ranged from slight (κ=0.044) to substantial (κ=0.779). The interobserver agreement varied extensively with regard to the presence of giant cells. The observed histologic features displayed no association with radiologic patterns or survival. Definite UIP and honeycombing findings in high-resolution computed tomography correlated with poor prognosis. A high level of interobserver variability was observed between pathologists, even in this well-defined cohort of IPF patients, which highlights the importance of multidisciplinary decision making in IPF diagnostics and stresses the need for a reassessment of the histologic criteria.