Flow cytometrically determined cellular DNA content has been measured on specimens from 101 patients affected by lung cancer (40 epidermoid cell carcinoma, 22 adenocarcinoma, 21 large cell carcinoma, 11 small cell carcinoma, and seven undifferentiated carcinoma), and one by mesothelioma. Ninety-eight of 102 (96%) patients with neoplastic disease evidenced the occurrence of at least one cytometrically aneuploid cell subpopulation. Fifty-five of 102 (54%) cases evidenced the occurrence of multiclonality, that is, the presence of more than one aneuploid stem cell line. However, the incidence of multiclonality in lung carcinoma was statistically different in surgical cases (where multiple site sampling from the primary and lymph nodes was possible) in comparison to the nonsurgical ones (e.g., bronchial washing): 48/77 (62%) and six of 24 (25%), respectively. Therefore, only the 77 surgical patients were used for further analysis. The cases were classified according to the DNA index (DI) in the following way: Group A (tumors with one or more stem lines with DI ranging from 1 to 2) and Group B (tumors with at least one stem line with DI less than 1 or greater than 2). A significant correlation has been found between the cytometric ploidy condition so defined (Groups A and B) and the tumor mass doubling time (DT), Group B being associated with fast growing tumors (DT lower than 90 days). A statistically better 12-month survival rate (5-year maximum follow-up) was observed in Group A (88%) in respect to Group B (47%) and is evident in the patient survival time course. A better prognostic indication can be achieved by stratifying the patients according to both the cytometric ploidy condition and the tumor DT. Flow cytometric data can usefully contribute to the prognostic assessment of lung carcinoma provided that representative cellular material is collected by multiple site sampling.