Magnetic resonance imaging (MRI) has recently been proposed as a method to monitor, noninvasively, temperature, blood flow, and cell metabolism during oncologic hyperthermia (HT). To heat and "image" simultaneously, it is necessary to combine a HT device and a MRI unit. As a demonstrative example of the problems associated with implementing such a system, a mini-annular phased array hyperthermia applicator was combined with a 0.5-T whole body MRI unit. With the aid of filters, baluns, and switches, the HT applicator and the MRI unit were made compatible. The overall system was tested using a muscle-equivalent, cylindrically shaped polyacrylamide gel phantom. No interference between the HT device and the MRI unit was observed. Noninvasive temperature images, with a resolution better than 1 degree C/cm, were obtained from images of molecular diffusion recorded before and during heating.