Between January 1982 and December 1986, 54 patients with squamous cell carcinoma of the supraglottic larynx were treated for cure with radiation therapy alone; all had a minimum 2-year follow-up. For 31 patients, high-quality pretreatment CT scans were available for review. Scans were retrospectively evaluated by a single radiologist for the percentage of preepiglottic space involvement by tumor, the percentage of paralaryngeal space involvement, and the total tumor volume in cubic centimeters (cc). Volume was measured using a computer digitizer for each CT slice showing tumor. Results showed a significant difference in local control for tumors less than 6 cc versus tumors greater than or equal to 6 cc (15/18 or 83% vs 6/13 or 46%; p = .038). This difference was independent of both T stage and primary site. For T3 lesions, there was a trend toward decreased local control with increasing percentage of preepiglottic space involvement (0-25% = 85% local control; greater than 25% = 60% local control; p = .384); in most cases, however, the amount of preepiglottic space involvement reflected tumor volume (0-25%, 8/12 or 67% of tumors were less than 6 cc; greater than 25%, 7/10 or 70% of tumors were greater than or equal to 6 cc). There was no clear association between degree of paraglottic space involvement and local control. Tumor volume did not correlate with complications of treatment. Because of the limited number of patients in the current study, conclusions should be considered preliminary. However, the data do suggest that pretreatment CT findings, particularly tumor volume, can predict the likelihood of local control with radiation therapy alone for squamous cell carcinoma of the supraglottic larynx.