Real time ultrasound and MR imaging cannot image tissues within cryosurgical iceballs. The ability of X-ray CT to both resolve the ice interface and continue to image tissue within a cryosurgical iceball has, for the most part, been ignored in the literature. Phantom studies of the change in Houndsfield unit with temperature for distilled water, saline-gelatin mixture and calf liver-gelatin suspension have been performed. We found a decrease in HU upon freezing with no drop with further cooling. This change occurred over a range of -7° and was found to be approximately 85 HU upon freezing, regardless of the initial (prefreeze) value. We observed no correlation between HU and temperature deep within the iceball. X-ray CT should be considered for the monitoring of iceball growth during cryosurgery, since it is able to resolve the three-dimensional geometry of the iceball and image the anatomy within it.