To evaluate the effect of various bowel contrast material concentrations and subtraction software on size measurements of well-defined polyp lesions in a colon phantom at CT colonography. Repeated scanning and a precise reference standard required the use of a colon phantom in which 21 polyps were randomly distributed. Two readers who had each reviewed computed tomographic (CT) colonographic images from more than 100 cases evaluated polyp size on images obtained when the phantom was partially filled with varying concentrations of contrast material, scanned by using CT colonography, and subjected to electronic subtraction cleansing. The single largest dimension was recorded for each reader for a randomized series of polyps. These measurements were compared with a reference standard that was based on a combination of the manufacturer's polyp size specifications and the subsequent verification of these sizes by an independent consensus panel. Six weeks after initial observations, readers evaluated images of the phantom scanned without the presence of contrast material. Polyp size estimations for the two readers for each series were compared with the reference standard to obtain a mean absolute measurement error for each reader for each series. Data for each reader were compared by using a nonparametric Kruskal-Wallis analysis of variance test. A pair-wise comparison of the experimental and control series was then performed by using the Dunn post hoc test. Contrast material dilutions resulting in an average attenuation of less than 500 HU resulted in complete subtraction and the absence of streak artifacts. There was no statistically significant difference between the average measurement error for contrast attenuations between 300 and 500 HU when compared with that of control. Streak artifact was noticeable for the highest dilution (mean, 840 HU). No statistically significant differences were observed for series in which cleansing software was used in the absence of bowel contrast material. The combination of electronic cleansing and bowel contrast enhancement in the range of 300-500 HU results in no substantial change in readers' estimations of polyp size at CT colonography. Copyright RSNA, 2005