Target volumes in the thorax and abdomen are commonly computed tomography (CT) scanned during light respiration. In this article, we analyze the distortions introduced in helical scanning of moving objects. Objects of known geometry are placed on a moving sled and scanned in a multirow helical CT scanner. The motion of the sled approximates the magnitude and velocity of organ movement in patients during light respiration (amplitude 1 cm, period 4 seconds). Scans of the phantom are obtained in high speed mode at incremental phases of respiration, and the resulting images are compared with scans obtained when the phantom is static. Computer simulations of the scan process are also performed to interpret the results and extend the analysis to a greater range of parameters in scanning, motion, and object size. Resulting scans show that spherical test objects can be shortened by as much as 2 cm or twice the periodic motion amplitude. Object shape was significantly distorted, and the geometric center of the object was displaced by as much as +/-0.8 cm. Computer simulation results qualitatively agree with the experimentally observed phantom images. These simulations predict that the effect is clearly observable even if the amplitude is decreased to 0.5 cm. Implications of scanning moving objects on treatment planning are discussed.