Background: Body mass index (BMI) is linked to an increased risk of cancers and a poorer prognosis. However, the evidence on the relationship between high BMI and metastatic castration resistant prostate cancer (mCRPC) is not entirely consistent and the data are conflicting. The aim of this observational, retrospective, multicenter study was to evaluate the correlation between BMI and progression-free survival (PFS) and overall survival (OS) in patients (pts) with mCRPC treated with chemotherapy. Methods: We collected mCRPC pts who received docetaxel 75 mg/m2 every 21 days as first line therapy in6 Italian Cancer Centers from 2005 to 2015. We classified BMI group according with the World Health Organization definition : normal weight BMI < 25kg/m2, overweight 25 ≤ BMI < 30kg/m2, and obese BMI ≥ 30kg/m2. Baseline characteristics and treatment information has been recorded in an anonymized excel file. Results: We collected 113 pts with a median age of 70.7 years (62 to 87) at the time of mCRPC diagnosis. In our cohort 33.6% of pts were normal weight at the HRPC diagnosis, 50.9% were overweight, and 15.5% were obese. At the baseline, 83.3% of obese pts had at least 1 co-morbidity versus 64.9% of normal/overweight pts. Moreover, 27.8% of obese pts needed a docetaxel dose reduction versus 17.5% normal/overweight pts. Only a non-significant trend for the detrimental effect of high BMI on PFS and OS has been documented. Median PFS in obese pts 6.4 months vs 7.0 months in of normal/overweight pts (p = 0.439); median OS in obese pts 38.8 months vs 43.4 months in of normal/overweight pts (p = 0.157). Conclusions: The relationship between BMI and mCRPC is extremely complex and unclear. Even if in this preliminary analysis we failed to confirm a significant association between BMI and survival, the data suggests that obesity may be associated with a lower tolerance to chemotherapy. Overall, we expected to enroll approximately 500 pts and data collection is currently ongoing.