PURPOSE: To investigate the relationship between the bulb of the penis and the peak of the urethrogram, and to compare this measurement with the ischial tuberosities (ITs) to peak distance. METHODS AND MATERIALS: Pelvic CT scans from 50 consecutive patients with localized prostate cancer were analyzed to identify the penile bulb. Each patient was required to undergo retrograde urethrography during CT-based treatment planning with 3-mm slices. The peak of the urethrogram was defined as the last CT slice in which the contrast dye in the urethra could be visualized. Measurements were taken from the slice containing the most superior aspect of the penile bulb to the last slice of the urethrogram peak. The superior aspect of the penile bulb was defined as the CT slice nearest the peak that contained a bulbous structure at the base of the penis. This distance was defined as the bulb-peak distance. Similarly, the IT-peak distance was recorded for comparison. RESULTS: The mean bulb-peak and IT-peak distances were calculated for 47 of 50 patients. The peak of the urethrogram was unable to be evaluated in 3 patients. The mean, median, and range bulb-peak distance was 2.4 mm (SD 1.8), 3 mm, and 0-6 mm, respectively. The mean, median, and range IT-peak distance was 20.1 mm (SD 6.6), 21 mm, and 6-33 mm, respectively. No patient had the bulb located above the apex of the urethrogram. CONCLUSION: The bulb of the penis is a relatively consistent soft-tissue landmark compared with the ITs and is located an average of 3 mm below the peak of the urethrogram. Therefore, the bulb of the penis is another landmark for the identification of the prostatic apex and is less invasive than retrograde urethrography.