To evaluate the relationship between cell proliferation and apoptosis during progression of lung carcinomas, immunohistochemistry for proliferating cell nuclear antigen (PCNA) and the in situ end labelling (TUNEL) method for identifying apoptotic bodies were performed on paraffin sections from 135 lung carcinomas. These results were correlated with the corresponding tumor volumes as a model of disease progression in lung tumors. We found that, with increasing tumor volume, the proliferation rate decreased significantly, whereas the apoptotic rate increased. There was no relationship between apoptotic and proliferative indices except in carcinomas with a tumor volume between 51 and 100 cm3. These data suggest that progression of lung carcinomas, i.e. the increase in tumor volume, is accompanied by an increase in apoptosis rather than an increase in cell proliferation.