Background:Both polycythemia and high leukocyte count are associated with the risk of cardiovascular disease. However, it remains to be determined whether polycythemia and high leukocyte count show synergistic increasing effects on cardiometabolic risk.Methods:Cardiometabolic risk was evaluated by cardiometabolic index (CMI) and metabolic syndrome in a cohort of middle-aged men (n= 11,140) who underwent annual health check-up examinations. The subjects were divided into three tertile groups by hemoglobin concentration or leukocyte count in peripheral blood, and their relations with CMI and metabolic syndrome were investigated. A new index, named hematometabolic index (HMI), was defined as the product of hemoglobin concentration (g/dL)-minus-13.0 and leukocyte count (/μL)-minus-3000.Results:When the subjects were further classified by tertiles for hemoglobin concentration and leukocyte count into nine groups, the odds ratios for high CMI and metabolic syndrome of the group categorized in the highest (third) tertiles for both hemoglobin concentration and leukocyte count versus the group of the lowest (first) tertiles for both of them were highest among the nine groups. In receiver-operating characteristic (ROC) analysis for relationships of HMI with high CMI and metabolic syndrome, areas under the ROC curves (AUCs) were significantly larger than the reference level and tended to be smaller with an increase in age. In subjects from 30 to 39 years of age, the AUC for the relationship between HMI and metabolic syndrome was 0.707 (0.663–0.751) and the cutoff of HMI was 9850.Conclusions:HMI, reflecting hemoglobin concentration and leukocyte count, is thought to be a possible marker for discriminating cardiometabolic risk.