Aspiration of particulate matter is a well-recognized complication in debilitated patients at autopsy but is not widely recognized in surgical pathology material. We have encountered a surprising number of cases on biopsy or resection specimens, and most were unsuspected clinically and pathologically. This study was undertaken to clarify clinical and pathologic features that facilitate the diagnosis of food/particulate matter aspiration pneumonia. Fifty-nine patients were identified with an average age of 57 (range 26 to 85), and a male/female ratio of 2:1. Common presenting symptoms (information available in 36 cases) included dyspnea (14), fever (9), and cough (6). A history of recurrent pneumonia was present in 9. Radiographic data were available in 34 cases. Bilateral infiltrates or nodules were found in 17 cases, whereas the changes were unilateral in 17. Solitary nodules clinically suspicious for neoplasm were present in 13. Aspiration was suspected clinically in only 4 of the 45 cases in which the clinical impression or differential diagnosis was stated. Predisposing factors for aspiration were identified in 32 patients, including esophageal or gastric causes (19), drug use (10), and neurologic conditions (6). Histologically, bronchiolitis obliterans-organizing pneumonia was present in 52 (88%) cases, usually in combination with multinucleated giant cells, acute bronchopneumonia/bronchiolitis, and/or suppurative granulomas. Foreign material was identified in all cases, including most commonly vegetable or food remnants (54 cases), and less often talc or microcrystalline cellulose (7), crospovidone (4), and kayexalate (2). Particulate matter aspiration pneumonia is a more common cause of lung infiltrates and nodules than generally appreciated. The diagnosis should be suspected when multinucleated giant cells, acute bronchopneumonia/bronchiolitis, and/or suppurative granulomas are found in a background of bronchiolitis obliterans-organizing pneumonia. The presence of foreign material confirms the diagnosis.