The role of interstitial implantation in the radiotherapeutic treatment of base of tongue carcinoma remains controversial. At the University of Florida, essentially all patients with base of tongue cancer have been managed initially by radiation therapy (with or without neck dissection) with operation reserved for radiation therapy failure. Eighty-four patients with invasive squamous cell carcinoma of the base of the tongue were treated with continuous-course external-beam irradiation without interstitial implantation between October 1964 and July 1986. Treatment was administered once-a-day in 59 patients and twice-a-day in 25 patients. The median follow-up was 99 months (range, 25-284 months). No patient was lost to follow-up. Local failure occurred in 1/9 patients (11%) with T1 lesions, 3/30 (10%) with T2, 6/31 (19%) with T3, and 9/14 (64%) with T4. If one excludes from the local control analysis those patients who died of intercurrent or metastatic disease within 2 years with their primary tumor continuously controlled, then the rates of local control are as follows: T1, 3/4; T2, 22/25 (88%); T3, 20/26 (77%); T4, 5/14 (36%). An improved local control rate for T4 tumors was noted with twice-a-day fractionation. Eighty-eight percent of N0-N1 necks and 79% of N2-N3 necks were treated successfully by irradiation with or without planned neck dissection. Five-year rates of continuous disease control above the clavicles were as follows: Stage I-II, 100%; Stage III, 72%; Stage IVA, 78%; Stage IVB, 44%. Five-year absolute and relapse-free survival rates for the entire group were 43% and 58%, respectively. The incidence of bone exposure was 6%, and that of soft-tissue necrosis was 19%. In all but one case, the complication was mild to moderate in severity and healed with conservative management. These results compare favorably with those recently published in the literature supporting moderate-dose external-beam irradiation combined with interstitial implantation. We conclude that interstitial implantation is not essential for the successful radiotherapeutic treatment of base of tongue carcinoma.