Patients with implanted pacemakers and defibrillators are routinely cautioned regarding exposure to environmental magnetic fields because such exposure may interfere with device function. Previous reports have confirmed interference with bingo wands, stereo speakers, and various workplace sources. In the 4 patients in this report, we document inadvertent alteration of the tachyarrhythmia detection function of implantable cardioverter-defibrillators (ICDs) that occurred in health care settings because of deliberately applied magnetic fields. Three of these patients had pectorally implanted ICDs that may have been confused with pacemakers, and 2 patients had undergone office surgical procedures at which time a magnet had been applied over the device. These events stemmed from (1) potential confusion by health care workers about the nature of the implanted device and (2) unique features in a specific manufacturer's defibrillator. We recommend the following steps to avoid such problems: (1) when device programming hardware and trained personnel are readily available, the patient's device should be interrogated and reprogrammed before and after any procedure involving electrocautery; (2) patients with ICDs should be monitored during device inactivation because they are unprotected from potentially life-threatening arrhythmias during this period; and (3) if the clinical situation does not allow device interrogation and reprogramming, the patient should be monitored electrocardiographically during magnet application and the device interrogated as soon as possible after magnet removal.