Carcinoma in situ of the glottic larynx is a rare malignancy, comprising only 1% to 2% of laryngeal biopsy. The management of these lesions remains controversial. In this article, the authors present results and analysis of the management of 20 patients having these lesions treated by radiotherapy. A total of 20 patients who had carcinoma in situ of the glottis were treated with curative intent with radiotherapy at the Veterans Administration Medical Center, Minneapolis, Minnesota, U.S.A. from November 1972 through March 1993. Before radiotherapy, eight patients had only biopsies of the glottic larynx, five had one stripping procedure, and seven patients had two or three stripping procedures. Of the 20 patients, 19 are available for retrospective analysis, with a minimum follow-up of 4 years and a median follow-up of 7.3 years. Treatment was given to all patients by megavoltage units (cobalt 60, 2; 4-mV linear accelerator, 18). Patients were treated using bilateral opposed wedged 6-cm x 6-cm fields, with a dose of 1.75 Gy per fraction to a total median dose of 68.4 Gy in 40 fractions over 56 elapsed treatment days. The Kaplan-Meier method was used for survival analysis. The 4-year disease-free survival was 95% after irradiation. The actuarial disease-free survival for 5 and 10 years was 95%. Surgical salvage failed in one patient who had a laryngectomy for recurrent invasive squamous cell carcinoma at 34 months after radiotherapy. Two patients developed severe shortness of breath because of laryngeal edema, which required tracheostomy. Sixteen patients reported subjective significant improvement of their voice quality, and four reported no change. Radiotherapy is an excellent choice for patients who have recurrent carcinoma in situ after undergoing stripping procedures or for those patients who live too far from a medical facility to allow close follow-up and repetitive surgical procedures. These results are comparable to those of other investigators who used radiotherapy with higher dose per fraction.