Purpose:To assess the effect of a decrease in tube voltage from 120 kVp to 100 kVp on dose, contrast-to-noise ratio (CNR), and three-dimensional (3D) image quality in patients undergoing computed tomographic (CT) colonography as well as to determine how these changes are affected by patient size. Materials and Methods:This HIPAA-compliant and institutional review board-approved retrospective study included 63 consecutive patients who underwent CT colonography and who waived informed consent. Scanning was performed with patients in the supine (120 kVp) and prone (100 kVp) positions, with other parameters unchanged. Volume CT dose index (CTDI vol ), dose-length product (DLP), image noise, attenuation of selected materials, and CNR were compared with the Wilcoxon matched-pairs signed rank test. Two readers blinded to tube voltage independently assessed 3D endoluminal image quality. The k coefficients were calculated for interobserver agreement. Average image quality ratings were compared with the Wilcoxon signed rank test. All recorded data were stratified by patient anteroposterior diameter to determine effects of patient size. Results:Decreasing tube voltage from 120 to 100 kVp resulted in a 20% decrease in CTDI vol (P , .001) and a 16% decrease in DLP (P , .001). Image noise increased by 32% (P , .001). Mean attenuation of tagged fluid increased from 395 to 487 HU (P , .001). There was no change in mean CNR of tagged fluid (17.1 at 120 kVp, 16.8 at 100 kVp; P = .37), regardless of patient size. The 3D image quality decreased slightly from a median score of 5 out of 5 to 4 out of 5 (P , .001). There was substantial interobserver agreement. Conclusion:A decrease in tube voltage from 120 to 100 kVp results in a significant decrease in radiation dose but only a minimal decrease in 3D image quality at all patient sizes.q RSNA, 2012