Flocking is a widely used industrial process in which short lengths of synthetic fibers are applied to backing fabric to produce plush material. In response to an apparent outbreak of interstitial lung disease in flock workers, the Centers for Disease Control hosted a clinical-pathological workshop to identify the defining characteristics of the disease and possible etiologic agents. Six pathologists reviewed 15 biopsies of 15 cases (out of a clinical caseload of 20 patients) and assessed the pattern, extent and degree of pulmonary inflammation, fibrosis, and other changes. A consensus clinical-pathologic diagnosis was reached for each patient and correlated with clinical and radiologic findings. Four of eight open lung biopsies and one of seven closed (transbronchial) lung biopsies demonstrated a characteristic pattern to which the descriptive terminology lymphocytic bronchiolitis and peribronchiolitis with lymphoid hyperplasia was applied. The other biopsies showed nonspecific inflammatory changes, airspace organization, and diffuse alveolar damage. One open lung biopsy demonstrated respiratory bronchiolitis with lymphoid hyperplasia. None of the lung biopsies showed more than mild interstitial fibrosis and no granulomas were identified. The consensus of the workshop was that lymphocytic bronchiolitis and peribronchiolitis with lymphoid hyperplasia was a characteristic and distinctive pattern of injury in the flock workers' lung biopsies. Although the etiology of this disease remains undefined at present, the injury pattern and environmental studies suggest a chronic immunologic response to inhaled material.