Radiotherapy treatment planning needs optimum definition of the target volume in its relative position to normal tissue. The motion of the kidneys during respiration has not been well quantified. They move in a tilted coronal and sagittal plane. Using fast MRI while patients held their breath we quantified the movements of the kidneys. Fourteen patients volunteered for the study. Nine MRI images of the kidneys for one volunteer were done: three in the axial plane (all in deep inspiration) and six in the coronal plane (three in deep inspiration, three in deep expiration). The maximal vertical motion of the superior pole from its end-expiratory to its end-inspiratory position is 39 mm (43 mm for the inferior pole). In deep inspiration or deep expiration the positions of the right and left kidneys appear reproducible. The mean deviation of kidney movement is less than 4 mm in all three dimensions (range, 0-6.9). For tumors close to the kidney, we advocate respiration gated radiation therapy so as to minimize the movement of this very radiosensitive structure.