Purpose: To determine the value of fiducials in daily image-guided prostate targeting for proton therapy (PT), to compare intrafraction motion between two stabilization strategies (rectal saline and balloon), and to determine the respective impacts of these combined strategies on planning target volume (PTV) expansions and smearing margins. Materials and Methods: Forty patients were randomly selected from a pool of low-risk prostate cancer patients with intraprostatic fiducials treated with proton therapy between 2006 and 2012, including 20 with intrarectal saline or 20 with endorectal balloons for daily prostate stabilization. Daily pre-and post-treatment orthovoltage (kV) films and digitally reconstructed radiographs (DRRs) were analyzed to determine prostate interfraction displacement, intrafraction motion, daily residual setup error in three axial dimensions (anterior-posterior, superior-inferior, and left-right), necessary population PTV expansions using van Herk's formula (2.5R þ 0.7r), and smearing margins. Results: Interfraction displacement population means did not differ significantly in either treatment group. Intrafraction displacement population means in the anteriorposterior direction were significantly smaller with balloons than with saline. With fiducial markers to account for interfraction motion, PTV margins could be reduced by 4.0, 4.2, and 2.3 mm in the anterior-posterior, superior-inferior, and left-right directions, respectively, in saline-treated patients, and by 6.3, 6.8, and 0.8 mm in each direction, respectively, in balloon-treated patients. With fiducials, PTV margins were smaller using rectal balloons compared with saline: 2.3 vs. 3.6 mm in the anterior-posterior direction, 2.7 vs. 3.4 mm in the superior-inferior direction, and 1.1 vs. 2.0 mm in the leftright direction. The maximum smearing margin in balloon patients were 10.7 mm. Conclusions: Fiducial markers are valuable for reducing the PTV expansion necessary to account for interfraction displacement. Rectal balloons were more effective than saline in decreasing intrafraction prostate motion, thereby permitting smaller PTV expansions to reduce the amount of normal tissue included in the target volume.