Antigen presenting S100 positive dendritic cells have been quantified in normal trachea, lung, bronchial lymph nodes, 130 lung tumors and in 100 lymph nodes regional to tumor. Dendritic cells are rarely seen as intraepithelial components of the normal bronchial mucosa, but are more commonly observed in the perivascular lymphoid tissue of the submucosa and in the alveolar septae of normal lung parenchyma (6 +/- 4.85 cells/HPF). The density of these dendritic cells is marked in histologically normal bronchial lymph nodes. Bronchioalveolar (Alveolar II), well and moderately differentiated squamous cell carcinomas contain the highest density of S100 positive dendritic cells, while small cell lung cancer and poorly differentiated squamous cell carcinoma show the lowest density. Regional lymph nodes to lung tumors with lymphocytic predominance and active germinal centres show the highest density of dendritic cells, while unstimulated lymph nodes contain the lowest number of S100 positive dendritic cells. Tumor infiltrating lymphocytes are marked in and around lung tumors with the higher density of dendritic cells. Survival of patients whose tumors contain high density of S100 positive dendritic cells is more favourable compared to tumors with low density of these cells. It is concluded that the density of the antigen presenting S100 dendritic cells in lung tumors is related to subtype, and tumor differentiation. A high dendritic cell density is associated with enhanced patient survival.