DNA content by flow cytometry was assessed in 47 cases from a series of 130 patients with non-small cell lung carcinoma (NSCLC) given radiation therapy postoperatively. This was done in an attempt to identify which patients might benefit, or not benefit, from postoperative radiotherapy. From archival formalin-fixed paraffin-embedded specimens, 16 of the 47 cases (34%) had DNA diploid tumors while 31 cases (66%) were nondiploid. A diploid DNA content was significantly related to improved overall survival (P = 0.0061) and tumor-free survival (P = 0.0167) but not with frequency of tumor recurrence within the irradiated field. Histological grade was not significantly related to overall survival, tumor-free survival, or frequency of in-field tumor recurrence. DNA content was found in this study of NSCLC patients irradiated postoperatively to be a useful marker for predicting survival but not for predicting local recurrence.