Few studies have investigated more than one cell-biological parameter in bronchioloalveolar epithelial hyperplasia (BEH) and bronchioloalveolar carcinoma (BAC). The authors have examined argyrophilic nucleolar-organizer regions (AgNORs) and DNA status in surgically resected formalin-fixed paraffin-embedded specimens, 27 BEH and 62 well-differentiated adenocarcinomas, including 30 BAC. The authors measured the mean AgNOR count in 200 nuclei from sections of these regions. The authors also quantified DNA distribution in more than 200 cancer cells from sections of these regions, stained with 4',6-diamidino-2-phenylindole, using a microspectrophotometer. Fourteen lesions were interpreted as atypical BEH. The mean number of AgNORs per nucleus in BEH was 1.25 to 2.63. The mean number of AgNORs was significantly lower in both typical and atypical BEH than in either the bronchial surface epithelial type or the bronchial gland cell type of well-differentiated adenocarcinoma (P < .05). The mean number of AgNORs in atypical BEH was intermediate between that in typical BEH and that in BAC. Quantitative DNA image analysis showed DNA aneuploidy in 2 of 18 BEHs and 18 of 52 well-differentiated adenocarcinomas. The incidence of DNA aneuploidy increased in this order: typical BEH (0%, none out of 10 lesions) through atypical BEH (25.0%, 2 out of 8 lesions), to adenocarcinoma (34.6%, 18 out of 52 cases). Thus, the incidence of DNA aneuploidy in atypical BEH (25.0%) was intermediate between typical BEH (0%) and BAC (30.0%). These results suggest that atypical BEH may be closely related to BAC.