A study of positional variation of anatomic structures in 38 patients undergoing CT in both the supine and prone positions prior to beginning radiotherapy is reported. Within the thorax, turning the patient to the prone position resulted in a ventral shift of hilar structures in 36 of 38 patients. In two patients, pulmonary metastases shifted ventrally and caudally in the prone position. In all cases reviewed, prone positioning produced a ventral shift of the heart and great vessels. Within the abdomen, prone positioning produced ventral as well as caudal shift of both the liver and spleen in 34 of 38 patients. In all of the patients, a ventral shift of the kidneys as well as caudal shift was noted. Little or no variation of position was noted in cervical or pelvic structures with changing body positions.