Non-small cell lung cancers (non-SCLC) differ from small cell lung cancers (SCLC) by many clinical features and prognosis. However, recent studies suggest that lung cancer heterogeneity frequently leads to the association of SCLC and non-SCLC in the same tumor. This phenotypic heterogeneity can be analyzed by immunohistochemistry using monoclonal antibodies (Mab) raised against differentiation related antigens. It may have clinical relevance inasmuch as the diversification of malignant cells is a well-known factor of tumor progression and may be due to chromosomal instability because inappropriate gene expression leads to the formation of antigens unrelated to cell lineage. Chromosomal instability in cancer leads to aneuploidy detectable by cell DNA content analysis. In a prospective study, we analyzed, in parallel, the expression of neuroendocrine related antigens by immunohistochemistry and the cell DNA content in frozen specimens from 40 patients who underwent complete surgical resection of primary non-SCLC in an attempt (a) to characterize the phenotypic heterogeneity and (b) to determine whether this heterogeneity is correlated with aneuploidy and clinical staging. Three Mabs were used in association as a marker of neuroendocrine antigen expression (S-L 11.14, MOC-1, and NE-25); reactivity of these Mabs in 9 SCLC and 3 lung carcinoid tissue sections was used as positive control. All SCLC and 2 of 3 lung carcinoids tested were homogeneously positive with Mabs S-L 11.14, MOC-1, and NE-25; 13 of 40 non-SCLC were homogeneously positive and 11 additional specimens focally positive with Mabs S-L 11.14, MOC-1, and NE-25. The frequency of this abnormal phenotype was significantly higher in poorly differentiated squamous cell carcinomas (x2 10.08; P < 0.005), in clinical stage III non-SCLC (x2 5.93; P < 0.02), and in tumors involving mediastinal lymph nodes (x2 5; P < 0.03). The percentage of cells in the modal DNA of G0-G1 phase was significantly lower in non-SCLC homogeneously positive with Mabs S-L 11.14, MOC-1, and NE-25 [27.4 ± 10.3% (SD)] in comparison with non-SCLC negative with these same Mabs [56.8 ± 21.3%; P < 0.01, Mann-Whitney U test]. We conclude that (a) mixed SCLC-non-SCLC differentiation is frequent and can be assessed by immunohistochemistry, (b) neuroendocrine differentiation in non-SCLC is mainly observed in poorly differentiated tumors and in advanced clinical stages, and that (c) this heterotopic phenotype is correlated with aneuploidy and has clinical implications.