The nuclear DNA content of 13 cases of atypical adenomatous hyperplasia (AAH) associated with adenocarcinoma, eight cases of small-sized well differentiated adenocarcinoma in which the tumors were less than 2.5 cm in size with no tumor recurrence within 5 yr after surgery, and eight cases of reactive type 2 pneumocyte hyperplasia of the lung was determined by cytofluorometry. Of the AAHs, four were solitary and composed entirely of cells of AAH (Group 1), and the remaining nine were in continuity with the focus of adenocarcinoma (Group 2). The average mean nuclear DNA content of the AAHs was significantly greater than that of reactive type 2 pneumocyte hyperplasia (p less than 0.005) and significantly smaller than that of adenocarcinoma associated with AAH (p less than 0.05). However, no significant difference was found in nuclear DNA content between AAH and small-sized well differentiated adenocarcinoma. Aneuploid stem lines were found in seven of the 13 (53.8%) AAHs, ten of the 13 (76.9%) adenocarcinomas associated with AAH, and five of the eight (62.5%) small-sized well differentiated adenocarcinomas but in none of the reactive type 2 pneumocyte hyperplasias. DNA histogram patterns I and II, in which aneuploid cells were less frequently present than in patterns III and IV, were more common in AAH than in adenocarcinoma associated with AAH and small-sized well differentiated adenocarcinoma. All of the reactive type 2 pneumocyte hyperplasia showed pattern I. As to the average mean nuclear DNA content, incidence of aneuploid stem lines, and DNA histogram pattern, there was no significant difference between groups 1 and 2 of the AAHs.(ABSTRACT TRUNCATED AT 250 WORDS)