Aim: Breast cancer is a heterogeneous disease. This study investigated the pretreatment prognostic significance of a new inflammatory index in hormone receptor positive (HR+) and human epidermal growth factor negative (HER2−) breast cancer patients. Methods: We retrospectively analyzed 80 patients diagnosed with HR+ and HER2− breast cancer between January 2014 and December 2018. We calculated prognostic inflammatory index (PII) = mean platelet volume (MPV) × neutrophil/lymphocyte. PII cut off was the best-predicted value by receiver operating characteristic (ROC) curve analysis. We used the Kaplan–Meier method to determine disease-free survival (DFS). We used the log-rank test to compare the DFS rates between the two patient groups. We performed a multivariate analysis by performing Cox regression analysis with prognostic factors defined in univariate analysis. Results: The median follow-up period was 38 (19–66) months. The 5-year survival was 91.3%. The 5-year DFS was 87.9%. The optimal cut-off value of MPV × neutrophil/lymphocyte ratio was determined as 22 by ROC curve analysis [area under the curve, AUC 735, HR % CI (confidence interval) 0.561–0.909, sensitivity 72.7%, specificity 70.4%]. The number of patients with PII ≤22 was 60, and the number of patients with PII >22 was 32. DFS was worse in the high PII group than in the low PII group (p = 0.001). Multivariate analysis revealed PII as an independent prognostic factor (p = 0.016). Discussion: In this study, we detected elevated MPV × neutrophil/lymphocyte ratio as an independent poor prognostic factor in operated HR+ and HER2− breast cancer patients. Prospective studies are needed to determine the prognostic significance of this index.