One hundred thirty previously untreated patients with invasive squamous cell carcinoma of the oral tongue received interstitial radiotherapy with curative intent using cesium needles. Ninety-nine patients were treated with interstitial radiotherapy alone and 31 patients received interstitial radiotherapy combined with external beam irradiation. The recurrence-free rates in the primary lesions were 94.4% (17/18) in T1, 91.2% (52/57) in T2, and 70.9% (22/31) in T3 lesions. The local recurrence-free rates with single-plane and two-plane implantation were good: 89.7% (70/78) and 85.7% (12/14), respectively. The rate of 64.2% (9/14) for volume implantation was significantly poorer (p < 0.05). It is evident that tumor volume is an important factor in the control of cancer following interstitial therapy. The overall incidence of ulceration of the tongue and mandibular complication was 20% (26/130) and 13% (17/130), respectively. Using both interstitial and external radiotherapy, the incidence was 22.5%, compared with 10.1% using interstitial radiotherapy alone. The mandibular complication incidence of 8.9% with single-plane implants was much lower than 20.8% for two-plane and 23.5% for volume implants. Interstitial radiotherapy is most suitable in T1 and T2 cases in which single-plane implantation is possible; for these patients interstitial radiotherapy, which has the advantage of preserving the structure and function of the tongue, should continue to be used in the future in spite of the progress in reconstructive surgery.