The ploidy status of the deoxyribonucleic acid of a malignant lung tumor provides additional information besides histologic grading and tumor staging according to lymph node infiltration and tumor metastasis. Ninety-nine surgical specimens from patients with non-small-cell lung carcinoma were investigated by flow cytometry. Deoxyribonucleic acid aneuploidy was found in 48% of the primary tumors. Patients with deoxyribonucleic acid-euploid tumors showed better survival (p < 0.01) than those with deoxyribonucleic acid-aneuploid carcinomas independent of tumor stage. Deoxyribonucleic acid ploidy status of the primary tumor was compared with that of N2 lymph node metastases in 29 cases. Seven samples showed a change from deoxyribonucleic acid aneuploidy in the primary tumor to deoxyribonucleic acid euploidy in the lymph node metastases. Survival was significantly better for patients with euploid primary tumors and lymph node metastases, followed by patients with deoxyribonucleic acid-aneuploid primary tumors and euploid lymph node metastases. Survival was poorest in patients with deoxyribonucleic acid-aneuploid primary tumors and lymph node metastases. It was observed that only the simultaneous determination of deoxyribonucleic acid ploidy of primary tumors and lymph node metastases permits accurate prognostic evaluation in case of lymph node infiltration.