Accelerators can be used for hadrontherapy, a cancer treatment in which the patient is irradiated with hadron beams. For this purpose, the Italian foundation TERA proposes the “cyclinac”, composed of a high-frequency linac which boosts the hadrons accelerated by a cyclotron. The main research goal is to reduce the length of the linac by achieving higher accelerating gradients, without compromising the reliability of the accelerator. In order to measure the high gradient limitation, a 3 GHz single-cell gauge cavity has been designed, built and high-power tested by TERA. Results from the first high power test are presented and discussed in this paper.