PURPOSE: To conduct a multi-institutional comparison of prostate brachytherapy preimplant dosimetry of Pd-103 and I-125. METHODS AND MATERIALS: Eight experienced brachytherapists submitted Pd-103 and I-125 monotherapeutic and boost preimplant dosimetry plans for central review. All 32 plans were calculated using the same transrectal ultrasound volumetric study. Seeds of any strength were acceptable, but were restricted to Theraseed Model 200 (Theragenics Inc., Buford, GA) and Oncura Oncoseed Model 6711 (Oncura, Plymouth Meeting, PA). The dosimetric analysis included evaluation of target volume, target to prostate ratio, target length, number of needles, seed activity, number of seeds, total activity, total activity divided by treatment planning volume, the use of extracapsular seeds, and average treatment margins (defined as the perpendicular distance between the prostate capsule and the 100% isodose line). Prostate coverage was defined in terms of V100/V 150/V200/V300 and D100/D 90/D50, whereas urethral dosimetry consisted of UV 100/UV150/UV200 and UD90/UD 50. RESULTS: The mean planning target volume to prostate volume ratio varied dramatically (mean 1.29, range 0.99-1.76) with the target length ranging from 3.5 to 4.5 cm. Although the prostate V100 was >95% in all cases, the V150 ranged from 29.9% to 92.1% and the V200 from 6.72% to 52.5%. The urethral V100 was 100% in all cases with six of the eight brachytherapists limiting the UV150 to <3%. However, the median urethral dose varied by up to 50%. Treatment margins also varied significantly (average 3.98 mm, range 0.32-7.68 mm). All brachytherapists used extracapsular seeds with five implanting >25% of the seeds in extracapsular locations (range 6.4-58.2%). In addition, significant variability existed in the number of needles, number of seeds, and seed strength. CONCLUSIONS: This study highlights the substantial variability that exists regarding target volume, seed strength, dose homogeneity, treatment margins, and extracapsular seed placement, although prostate brachytherapy prescription doses are uniform. The standardization of preimplant dosimetry is essential for meaningful multi-institutional comparisons of biochemical outcomes and morbidity.