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The pathologic findings in biopsy and subsequent explant specimens from 20 patients with usual interstitial pneumonia (UIP) were reviewed to refine histologic criteria for diagnosis, to identify factors that may confound diagnosis, and to assess the relationship of UIP and nonspecific interstitial pneumonia (NSIP). One case of NSIP was also identified and included for comparison. Surgical biopsies from 15 of the 20 UIP cases were diagnosed as UIP, whereas 5 showed only nondiagnostic changes. An important new observation is that areas resembling nonspecific interstitial pneumonia (NSIP-like areas) are present in the majority of UIP cases in both biopsy and explant specimens, and they are extensive in some. Ten of the 15 UIP biopsies were considered straightforward, with typical patchy interstitial fibrosis, honeycomb change, and fibroblast foci. Five cases were considered difficult because of prominent NSIP-like areas in two, extensive honeycomb change in one, superimposed diffuse alveolar damage in one, and superimposed bronchiolitis obliterans-organizing pneumonia in one. The most helpful feature for diagnosing UIP in difficult cases was the presence of a distinct patchwork appearance to the characteristic uneven or variegated parenchymal involvement along with evidence of architectural derangement. No explant showing UIP was preceded by biopsy findings of NSIP, and the one NSIP case appeared similar at biopsy and explant. NSIP or NSIP-like areas and UIP may reflect different mechanisms of fibrosis related either to different severity of injury or to different injuries.