Background: Stereotactic Body Radiation Therapy (SBRT) is a standard therapeutic option for men with prostate adenocarcinoma. There is a small body of literature characterizing a PSA bump after treatment with SBRT. Despite this, there is a paucity of data addressing rates of transient PSA increase immediately after SBRT treatment and what factors portend for this rare occurrence. This study reports outcomes on initial PSA after SBRT therapy for men who have received definitive SBRT for prostate cancer. Methods: Between May 2006 and February 2014, 921 patients with prostate cancer were treated with SBRT delivered via Cyberknife therapy. The mean age was 67 years old. 68.5% of patients were Caucasian, 17.4% African American, and 14.1% were another ethnicity. Patients were divided into prognostic risk groups with 44.7%, 43.5%, and 11.1% of patients falling in the low, intermediate, and high risk stratifications. Gleason scores were < 6 in 44.4%, 7 in 41.3%, and 8-10 in 14.2%. 37 patients also received supplemental external beam radiation (median dose 4500cGy) and 8.9% of patients received Androgen Deprivation Therapy (ADT) as part of their treatment regimen. Pre-treatment PSA was 0.8 – 205 ng/ml (median 6.1). Results: At 3 months’ follow-up, 2.8% of patients had a PSA elevated above their baseline value. 27.8% of patients age ≤50 (p<.0001) and 8.4% of patients age ≤60 (p=.001) experienced an increased in baseline PSA. African Americans were more likely to experience a transient increase in PSA over Caucasians (7.8% vs. 3.0%, p=.06). Patients treated with fewer beams were also more likely have a temporary rise in PSA (p=.056). Gleason Score, Risk grouping, prostate volume, ADT, and EBRT did not predict for rise on Pearson Chi-Square analysis. On multivariate analysis, only age ≤50 (p<.0001) portended for increased PSA at 3 months’ time. Conclusions: This represents the largest series evaluating elevation of PSA after definitive SBRT for prostate cancer. Temporary rise in baseline PSA after SBRT is rare. Despite that, a significant cohort of younger patients can experience transient elevation. Patient age ≤50 was found to be the only predictor of this otherwise rare occurrence.