The neutrophil-to-lymphocyte ratio (NLR), a marker of inflammation, has been reported to be a poor prognostic indicator in prostate cancer. Here we explore the use of the NLR to establish a simple prognostic score for men with metastatic castration-resistant prostate cancer (mCRPC) treated with docetaxel. METHODS: In the training cohort, the NLR and other known prognostic variables were evaluated among a cohort of chemotherapy-naïve patients treated with thrice-weekly docetaxel at the Princess Margaret Cancer Centre. Significant prognostic variables identified by univariable Cox regression were evaluated by the area under the receiver operating characteristic curves. Multivariable Cox regression was then used to derive a prognostic score where 1 risk point was assigned for each significant variable. The model was externally validated in a cohort of patients treated at the Royal Marsden. RESULTS: Three hudred fifty-seven patients were analyzed in the training cohort. Median age was 71 years, 12% had liver metastasis, and median overall survival (OS) was 14.7 months. Liver metastases, hemoglobin <12 g/dL, alkaline phosphatase >2.0× upper limit of normal (ULN), lactate dehydrogenase >1.2× ULN, and NLR >3 were associated with significantly worse OS in multivariable analysis. Four risk categories were subsequently established with 0, 1, 2, and 3-5 points. Two-year OS rates for these categories were 43%, 37%, 12%, and 3%, respectively. Area under the curve for the training cohort was 0.78 (95% CI, 0.72-0.84) compared with 0.66 (95% CI, 0.58-0.74) for the 215 patients in the validation cohort. CONCLUSIONS: This simple risk score provides good prognostic and discriminatory accuracy for men with mCRPC. Cancer 2014;120:3346–3352. © 2014 American Cancer Society.