Body tissue ablation is carried out by inserting a probe into a body of a living subject, urging the probe into contact with a tissue in the body, generating energy at a power output level, and transmitting the generated energy into the tissue via the probe. While transmitting the generated energy the ablation is further carried out by determining a measured temperature of the tissue and a measured power level of the transmitted energy, and controlling the power output level responsively to a function of the measured temperature and the measured power level. Related apparatus for carrying out the ablation is also described.