Background: Limited data are available regarding the relationship between body mass index (BMI) and metastatic castration resistant prostate cancer (mCRPC). Pro-BMI trial is an Italian, observational, retrospective study designed to evaluate the correlation between BMI and outcome, in terms of progression-free survival (PFS) and overall survival (OS), of mCRPC patients (pts) treated with first-line chemotherapy. Methods: We analyzed 212 consecutive mCRPC pts who received docetaxel 75 mg/m² every 3 weeks as first-line therapy in 10 Italian Institutions between 2003 and 2015. Data about baseline characteristics, treatment and outcome were collected in an anonymized dedicated database. Pts were stratified in 3 groups according to the World Health Organization BMI classification: normal weight (BMI < 25kg/m²), overweight (25 ≤ BMI < 30kg/m²), and obese (BMI ≥ 30kg/m²). PFS and OS were measured from the date of start of docetaxel: preplanned analyses aimed to compare obese vs. normal/overweight pts. Results: The median age at the time of start of docetaxel for mCRPC was 69 years (range 45-87). At baseline, 32% of pts were normal weight, 51% were overweight, and 17% were obese. 89% of obese pts had at least 1 co-morbidity versus 63% of overweight pts and 54% of normal weight pts (p = 0.002). PFS was not statistically different between obese (median 10.4 months) and normal/overweight pts (median 7.2 months, Hazard Ratio 0.84, 95%CI 0.58 – 1.22, p = 0.35). Similarly, OS was not statistically different between obese (median 27.7 months) and normal/overweight pts (median 24.4 months, Hazard Ratio 0.80, 95%CI 0.49 – 1.31, p = 0.38). Proportion of pts starting with full dose and needing dose reduction was slightly higher in obese pts (32.4%) vs. normal/overweight pts (21.5%), but this difference was not statistically significant (p = 0.17). Incidence of any hematologic toxicity was significantly higher in obese pts (69% vs 50%, p = 0.03). Conclusions: In this retrospective analysis, no significant association between BMI and outcome of pts receiving docetaxel for mCRPC was found. However, our results suggest that obesity could be associated with a lower tolerance to chemotherapy.