Conformal radiotherapy requires confidence that the images used for target delineation accurately reflect the pathological dimensions of the target. Radiosurgery, which is a conformal radiotherapy technique, is often used to treat brain metastases. The images of brain metastases can be affected by the method of image acquisition. A prospective study was undertaken to evaluate the effect of delay on CT images of brain metastases selected for radiosurgical treatment. A median delay from contrast administration of 65 min resulted in an increase in the volume of the metastases in 86% of cases when compared with the volumes of the same metastases determined from CT images acquired immediately following the administration of contrast medium. The magnitude of the increase in volume was sufficient to cause radiosurgery planners to select larger collimator sizes for radiosurgery plans based on the delayed CT images in 92% of cases. No significant intraobserver or interobserver variation was found in the group of radiosurgery planners. Differences in image acquisition may account in part for the differences in local control reported in the radiosurgical treatment of brain metastases.