AbstractThe cellular changes produced by radiation therapy were studied in cytologic specimens from seven patients with bronchogenic carcinoma: squamous‐cell carcinoma (four patients), adenocarcinoma (two patients), and small‐cell undifferentiated carcinoma (one patient). Cytologic samples were obtained from sputum and bronchoscopic examination in patients before and after they received a course of radiation. The cells were studied by light microscopy. An increase in the percentage of columnar cells was detected in five of seven patients independent of the latency of cytologic sampling posttherapy. The percentage of macrophages was unchanged in patients sampled at 15, 18, and 24 mo after the last radiation dose; leukocytes were decreased in relative frequency in the majority of patients studied. Following irradiation, moderately atypical and severely atypical metaplastic cells were increased in relative frequency (48% versus 15% and 19% versus 3%, respectively) and malignant cells were decreased (40% versus 22%) or absent in all patients, irrespective of the type of carcinoma. Nuclear vacuolation, nuclear enlargement, loss of chromatin texture, and rupture of the chromatinic rim were seen in atypical and cancer cells. Infiltration of cancer cells by leukocytes was observed in two of four patients with squamous‐cell carcinoma. In all treated patients, morphologic observations indicated that irradiation produces similar damage to “normal” bronchial epithelial cells although such changes are less apparent at longer time intervals followin