One of the worldwide key comparisons performed at present under the auspices of the International Committee for Weights and Measures (CIPM) covers the time-averaged ultrasonic power emitted by an ultrasonic transducer into water. The ultrasonic power value is particularly important in characterizing the output of diagnostic and therapeutic medical equipment. Nine national metrology institutes participate in the comparison. The time schedule is from July 1999 to November 2001. The aim is to demonstrate the international consistency of calibration and measurement certificates. An ultrasonic standard transducer is circulated and power measurements are to be made at frequencies of 2, 6 and 10 MHz and at five power levels in the milliwatt and watt ranges. The measurement procedures applied and the main details of the technical protocol will be discussed.