Forty open lung biopsies from patients with rheumatoid arthritis and possible "rheumatoid lung disease" were reviewed in an attempt to correlate histology with radiologic, physiologic, and prognostic variables. A wide variety of histopathologic features was seen, and primary and secondary patterns of injury were recognized. Five different groups based on histologic patterns were identified: pulmonary rheumatoid nodules, usual interstitial pneumonia (UIP), bronchiolitis obliterans with patchy organizing pneumonia (BOOP), lymphoid hyperplasia, and cellular interstitial infiltrates. The finding of rheumatoid nodules as the primary pattern imparted a uniformly good prognosis, whereas the pattern of UIP indicated a poor one. Patients with BOOP had a more favorable prognosis than did patients with UIP, as did patients with lymphoid hyperplasia and/or nonspecific cellular interstitial infiltrates. Consistent correlations between pulmonary function testing and roentgenographic and histologic findings were not found. The term "rheumatoid lung disease" is of no use as a histologic diagnosis because it encompasses a broad spectrum of morphologic changes that carry significantly different prognoses.