We describe the development of a three-dimensional in vitro organ culture model for bronchial carcinoma using bronchial mucosa organ cultures and three different human non-small cell lung cancer cell lines. During precultivation, bronchial fragments obtained as biopsies during routine bronchoscopy had regenerated a complete epithelial covering with a well-preserved organotypic architecture around a nucleus consisting of connective tissue. To create cocultures, different types of confrontation between tumor cells and organ cultures were applied. Histologic light microscopy and scanning electron microscopy were used in analysis. When tumor cells were confronted with completely epithelialized organ cultures, they showed a low incidence of attachment. When organ cultures were wounded before confrontation, tumor cells always attached to the wounded side and showed a progressive invasion into the stromal tissue. Measurements of the penetration depth of tumor cells into the organ cultures after different incubation times permitted the quantitative evaluation of invasion. Histologic studies revealed well-differentiated normal epithelium in spite of long culture periods. Histologic features of the tumors were those of an invasive undifferentiated carcinoma and showed marked similarities to the situation in vivo. The coculture model permits internal controls because it contains both normal human epithelium and human tumor cells in the same organotypic culture. Therefore it offers opportunities for various in vitro investigations on therapeutic and diagnostic modalities of lung cancer, as indicated in this paper by an example of photodynamic procedures with 5-aminolevulinic acid.