The electromagnetic compatibility (EMC) of 400 MHz radio communications is an issue of growing interest, especially the EMC of wireless medical telemetry (WMT) in clinical settings. Electromagnetic interference (EMI) by radiation noise emitted from switched-mode power supplies built into light-emitting diode (LED) lights is now of considerable interest for the safe operation of WMT. Moreover, the 400 MHz frequency bands that are allocated for WMT are also shared and used for other radio communication systems. In this work, we studied the EMI problem of WMT used in Japan. We experimentally investigated EMI with WMT caused by electromagnetic noise emitted from LED lights. In addition, we suggested countermeasures for these EMI issues and the management of an electromagnetic environment by a very simplified method.