From June 1989 to December 1993, a Phase I/II protocol using high dose-rate (HDR) interstitial brachytherapy for early stage node-negative squamous cell carcinoma of the mobile tongue was undertaken to assess the toxicity and efficacy of this modality. A total of 27 patients were treated: T1-10 patients, T2-15 patients, and T3-two patients. Seven fractions x 6.5 Gy of HDR brachytherapy were given on a twice daily schedule, with a minimum interval time of 6 h, over a period of 3.5 days. The actuarial tumor control probability after HDR brachytherapy was 53% at 5 years. Local control rates for the T1 and T2 tumors were lower than comparable historical controls treated at our institution using low dose-rate (LDR) radium or cesium needle implants and iridium wire implants. This is despite the fact that the HDR schedule was calculated by the linear quadratic formula to have equal tumor killing effects as 60 Gy in 6 days of LDR radiation. In addition, there was a trend towards higher incidence of severe complications for the HDR patients compared to historical controls treated with LDR. We would caution against the use of this schedule of HDR brachytherapy until further studies are done.