Background: MRI−TRUS fusion biopsy (FBx) use in the diagnosis of prostate cancer (PCa) results in a more accurate assessment of disease burden and has increasingly been incorporated into urologic practice. In addition, with more men choosing active surveillance (AS) and the reports of increased PCa aggressiveness with obesity, we wanted to study the impact of obesity on the risk of PCa progression in men on AS diagnosed and followed by MRI and MRI−TRUS FBx. Methods: A retrospective review was performed on a prospectively maintained database of all men who underwent MRI−TRUS FBx at our institution from January 2007 to May 2015. Patient demographics, clinical data, imaging, pathology, treatment and outcomes were recorded. Patients who enrolled on AS were stratified by BMI into normal weight (BMI 18.5−24.9), overweight (BMI 25.0−29.9), and obese (BMI ≥ 30.0). Statistical analysis was performed using SPSS software. Results: 204 men were enrolled in AS. Within the AS cohort, 51 (25%) had a normal weight, 101 (49.5%) were overweight, and 52 (25.5%) were obese. Age, BMI, PSA and mean estimated progression free survival time are described for each of these groups in Table 1. The overall rate of progression was 32.8%. Of the patients who progressed, 18 (26.9%) were normal weight, 32 (15.7%) were overweight and 17 (25.4%) were obese. On multivariate analysis, BMI was not a risk factor for AS progression, HR = 1.00 (p = 0.99, 95% CI = 0.95−1.06). Conclusions: There is evidence of increased risk of aggressive PCa specific death in obese patients. However, we demonstrate that in patients diagnosed by FBx, obesity does not confer an additional risk of progression on AS. This may be due to the improved characterization of cancer volume and grade by MRI−TRUS fusion biopsy. Further study is required to determine risk factors for AS progression in patients undergoing FBx. This research was supported by the Intramural Research Program of the National Cancer Institute, NIH, Medical Research Scholars Program.