Background: Body mass index (BMI) at diagnosis is associated with increased risk of fatal prostate cancer, but the link between BMI at mCRPC and cancer progression is less clear. Cachexia, often defined as involuntary weight loss > 5% over 6 months, is common in advanced cancers. The goal of this study was to examine the link between BMI at mCRPC and weight change as it relates to cancer progression, the outcomes of survival, and treatment use in a single-institution setting. Methods: 58 mCRPC patients treated at Tulane Hospital were identified, 41 of whom had an overweight BMI at mCRPC (BMI > 25) and 17 with normal BMI at mCRPC (BMI < 25). All patients had a confirmed prostate cancer death. Survival, treatment history, and percent weight change were compared according to BMI status. Rate of percent weight change was defined as the change in weight per day, from date of mCRPC diagnosis to the last treatment stop date or death date (“mCRPC days”). Linear regression, overall survival (OS), and nonparametric analyses were performed. Results: There was no significant difference between the normal and overweight BMI groups in overall survival, from date of diagnosis to death (median = 1835 days vs. 2710 days respectively). Additionally, the difference in survival from mCRPC to death was not statistically significant (median = 630 days vs. 799 days, p = 0.115). Use of Taxotere was not significantly different (47% vs. 68% respectively); however, overweight patients (n = 28) more likely received Abiraterone than normal BMI patients (n = 2) (p-value = 0.0001). The rate of percent weight change was significantly different for normal and overweight patients (mean = –0.050%/day vs. –0.019%/day, p = 0.003). Linear regression analysis showed that mCRPC days had a significant effect on percent weight change (p = 0.0109), and this effect was not significantly different between BMI groups (p = 0.6991). Conclusions: Survival after mCRPC was not significantly different between BMI groups. We observed a significant effect of mCRPC days on percent weight change, with a similar effect for both BMI groups. This outcome is expected, as more time would allow for greater weight changes to occur. Larger studies are needed to fully evaluate these observations.