Objective: To assess the impact of CT slice index and thickness ͑3 mm versus 5 mm͒ on ͑i͒ prostate volume, dimensions, and isocenter coordinates, ͑ii͒ bladder and rectal volumes, and ͑iii͒ DRR quality, in the treatment of prostate cancer. Methods: 16 patients with prostate cancer underwent two planning CT-scans using 3 and 5 mm slice index/thickness. Prostate, bladder, and rectum were outlined on all scans. Prostate isocenter coordinates, maximum dimensions, and volumes were compared along with bladder and rectal volumes. Bladder volumes and maximum diameters were further investigated using a second observer. A comparative analysis of DRR quality was conducted as well as a dosimetric analysis using DVH. Results: The differences in measurements of prostate volume, isocenter coordinates and maximum dimensions between the 3 and 5 mm scans, were small and not statistically significant. Similar finding was seen for rectal volume. However, bladder volume was always larger on the 3 mm scan ͑mean differenceϭ27.9 cc; SEϭ4.8 cc; 95% CI: 17.7-38.2 cc; pϽ0.001) and the findings were reproduced with the second observer ͑mean differenceϭ31.9 cc; SEϭ4.7 cc; 95% CI: 21.9-41.9 cc; p Ͻ0.001). The differences in volume are caused by a slight increase in ͑1͒ the measurement of the longitudinal dimensions on the 3 mm scans, and ͑2͒ the slice by slice measured bladder area on the 3 mm scans. The latter is due to partial volume effect. The 3 mm DRR were slightly better than the 5 mm DRR. The bladder DVH differed significantly in some patients. Conclusion: Bladder volume is significantly larger on the 3 mm scans. Differences in contoured areas may be accounted for, in part, by the partial volume effect.