Purpose: The purpose of this study was to compare the electroacoustic outputs of frequency-modulated (FM) systems coupled to hearing aids. Method: Electroacoustic performance of FM systems coupled to hearing aids was determined for 3 FM receivers: body-worn with neck loop, ear-level nonprogrammable, and ear-level programmable. Systems were evaluated using the FM-advantage approach suggested by the American Speech-Language-Hearing Association's "Guidelines for Fitting and Monitoring FM Systems" (2002). Output differences between the hearing aid and FM system were examined for typical input levels with a complex signal to verify an FM advantage and for high-input levels with a pure-tone signal to confirm similarity in maximum output. Results: Measurements from the FM receivers with neck loops showed significant low-frequency reduction and the most variable performance of the 3 types of systems. Less variable measurements were obtained with the ear-level FM receivers, and programmable FM receivers allowed for the most flexibility in obtaining an FM advantage. Findings of variability may be related to the type of system and the compression characteristics of the devices. Conclusion: Findings of great variability in electroacoustic performance of all systems support the need for electroacoustic evaluation, particularly when observing the effects of various settings on new technology. (Contains 2 tables and 6 figures.)