Objective: The hemoglobin, albumin, lymphocyte, and platelet (HALP) score has been shown to be an important prognostic marker in some tumor types. The aim of this study was to evaluate the prognostic impact of the preoperative HALP score, with the intent to develop a new prognostic index for patients with metastatic prostate cancer (mPCA) after cytoreductive radical prostatectomy (cRP).Methods: We retrospectively analyzed the data from 82 patients with mPCA after cRP in our institution. Of these patients, 70 patients were diagnosed with oligometastatic prostate cancer (oPCA). The main outcome measure was prostate-specific antigen (PSA) progression-free survival (PFS), which was assessed using Kaplan-Meier curves with log-rank statistics. In addition, univariate and multivariate Cox regression analyses were performed to determine the prognostic factors associated with PSA-PFS. The prediction accuracy was evaluated by assessing the area under the receiver operating characteristic (AUC) curve.Results: The median follow-up time for all patients was 17.47 months (range: 11.73-24.38 months). Based on the Kaplan-Meier curve analysis, it was noticed that a low preoperative HALP value (<32.4) was significantly associated with a decreased PSA-PFS in both the mPCA and oPCA subgroups (P < 0.001, P = 0.002, respectively). In addition, multivariate analysis predicted that a low HALP score was a common independent prognostic factor of an overall shorter PSA-PFS (HR: 0.352; range: 0.154-0.804; P = 0.013). However, among the different subgroups, a low HALP score (HR: 0.275; range: 0.116-0.653; P = 0.003) was confirmed to be an independent predictor of a shorter PSA-PFS in patients from the oPCA subgroup. Furthermore, the effective combination of the pathologic Gleason score (PGS) and the HALP score (HALPG) as a new index was found to be an independent risk factor. Also, the AUC of the HALPG score for PSA-PFS was observed to be higher than other conventional clinical indices.Conclusion: Overall, our results confirmed the HALP score as an independent prognostic factor for PSA-PFS in patients with mPCA or oPCA after cRP. Moreover, the new index, HALPG, also appeared to be an independent prognostic factor and was better than the HALP score. Importantly, it is evident that this new prognostic index has the ability to accurately identify patients at low, intermediate, and high risk of recurrence, thus easily allowing informed treatment decisions to be made.