A prospective randomized study to determine the effect of radiation field on the local control of early glottic cancer was performed. From May 1982 through December 1985, a total of 87 patients with early glottic cancer (T1N0M0) were treated at our department with an individualized wedge filter technique using a shell to improve the dose distribution and immobilization during radiotherapy with 4 MV X ray. Patients were randomly allocated to either treatment group A (radiation field size: 5 x 5 cm2) or B (6 x 6 cm2) using bilateral parallel opposed portals. Total radiation dose administered was 60 Gy in 30 fractions over a 6-week period. The 3-year recurrence-free survival rates were 93% in group A and 96% in group B (no significant difference). Comparison with historical control showed that both arms achieved better results. These results were mainly due to the improved local control of whole-length lesions of single vocal cords. Acute mucosal reaction (p less than 0.05) and persistent arytenoid edema lasting more than 6 months (p less than 0.02) were more frequently observed in group B than in group A. It is concluded that the critical factor in getting optimal results in the treatment of early glottic cancer is careful attention to the technique of treatment.