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A 2.4-GHz energy-efficient transmitter (TX) for wireless medical applications is presented in this paper. It consists of four blocks: a phase-locked loop (PLL) synthesizer with a direct frequency presetting technique, a class-B power amplifier, a digital processor, and nonvolatile memory (NVM). The frequency presetting technique can accurately preset the carrier frequency of the voltage-controlled oscillator and reduce the lock-in time of the PLL synthesizer, further increasing the data rate of communication with low power consumption. The digital processor automatically compensates preset frequency variation with process, voltage, and temperature. The NVM stores the presetting signals and calibration data so that the TX can avoid the repetitive calibration process and save the energy in practical applications. The design is implemented in 0.18- μm radio-frequency complementary metal-oxide semiconductor process and the active area is 1.3 mm 2. The TX achieves 0-dBm output power with a maximum data rate of 4 Mb/s/2 Mb/s and dissipates 2.7-mA/5.4-mA current from a 1.8-V power supply for on-off keying/frequency-shift keying modulation, respectively. The corresponding energy efficiency is 1.2 nJ/b·mW and 4.8 nJ/b· mW when normalized to the transmitting power.