Host defense mechanisms play important roles in suppressing the development and growth of tumors. It is known that S-100 protein-positive immature dendritic cells (S100DC), as antigen presenting cells (APC), and macrophages have roles in the immune responses to tumor growth. Mediators such as nitric oxide are also important in the surveillance against cancer. We examined the distribution of S100DC and CD68-positive macrophages (CD68MØ) immunohistochemically to compare the condition of apoptotic tumor cells in 69 patients with human non-small cell lung carcinoma. The expression of inducible nitric oxide synthases (iNOS) in tumors was also studied. Unlike macrophages, S100DC were distributed predominantly in cancer nests. In the areas with infiltration of 'many' S100DC (i.e. more than 10 DC/HPF), we found two distinct patterns of tumor infiltration: scattered and aggregated infiltration of DC in tumor nests. In areas of scattered S100DC distribution, only a few apoptotic tumor cells could be detected. However, in the areas of DC aggregations, apoptotic tumor cells were significantly more abundant (P = 0.0491). In contrast to S100DC, the distribution and density of CD68MØ were associated with iNOS expression of tumor cells (P < 0.0001), but not with distribution of apoptotic tumor cells. These findings reveal differences in the in vivo condition between S100DC and CD68MØ in tumors, and suggest there is a relationship between tumor-infiltrating S100DC aggregation and apoptosis in in vivo non-small cell lung cancers.