Twenty-seven patients with squamous cell carcinoma of the mobile tongue and floor of the mouth were treated with external beam and interstitial radiation. Good prognostic factors were T1N0, T2N0, superficial tumors, tumor shrinkage by 75% with external beam, and no apparent tumor clinically 2 months after treatment. On the other hand, T3N0, T1-3N1, and deeply necrotic tumors had a poor prognosis. We recommend using a flexible afterloading system to implant the initial local tumor volume (not just the residual nidus) that does not exceed 45 cm3. The minimum (reference) dose was prescribed to a surface 1/2 cm beyond the most peripheral rim of radioactive sources. For acceptable local control and complication rates, our suggested minimum (reference) doses are less than or equal to 7,500 rads for T1 (or a time-dose-fractionation [TDF] of 131-140), less than or equal to 8,000 rads for T2 (TDF of 131-140), and probably less than 8,500 rads for T3 (TDF of less than or equal to 150). These guidelines should be considered preliminary.