Pleural nodules or sheets of histiocytes, admixed with eosinophils, giant cells and other inflammatory cells are observed frequently in patients with spontaneous pneumothorax. This reaction, designated reactive eosinophilic pleuritis (REP), can closely resemble eosinophilic granuloma. Reactive eosinophilic pleuritis was found in pleural tissue of 22 of 57 patients with spontaneous pneumothorax in whom tissue was available for study. None of these patients had clinical or radiographic evidence of interstitial lung disease. Follow-up of 20 patients with REP ranged from 6 mo.-5 yr. None developed evidence of eosinophilic granuloma. EM of 1 case did not show the Langerhans granules characteristic of the histiocytes of the differentiated histiocytoses, including eosinophilic granuloma. Reactive eosinophilic pleuritis appears to be a nonspecific reaction to pleural injury and is not a prodromal feature of pulmonary eosinophilic granuloma.