We have retrospectively studied 53 patients with idiopathic pulmonary fibrosis and a histologic diagnosis of usual interstitial pneumonia and evaluated the prognostic significance of four individual histologic features (fibroblastic foci [FF], interstitial mononuclear cell infiltrate, established fibrosis, and intra-alveolar macrophages) using a semiquantitative scale of 0-6. An objective count of FF was also undertaken. Using weighted kappa coefficients, interobserver agreement between pathologists was moderate to good (0.56-0.76). Subjective and objective FF scores were strongly associated (R(S) = 0.88, < 0.00005). Mortality was independently linked to a high FF score, p = 0.006, and a low percent predicted carbon monoxide diffusing capacity (DL(CO)), p = 0.01. For pulmonary function, on univariate analysis, the strongest correlations were observed between increasing interstitial mononuclear cell infiltrate or FF scores and greater declines in forced vital capacity (FVC) or DL(CO) at 6 months. Multivariate models revealed that increasing FF scores were independently associated with greater declines in FVC and DL(CO) at both 6 and 12 months. Increasing interstitial mononuclear cell infiltrate scores were also independently linked to functional decline, but only at 6 months. These data suggest a reproducible method on biopsy for predicting rate of disease progression in patients with idiopathic pulmonary fibrosis.