Polycythemia vera (PV) is characterized by leukocytosis, thrombocytosis, and low mean capsular volume (MCV). No specific hematological findings are observed in patients with erythrocytosis of other origins. However, recombinant erythropoietin (EPO) increases the number of mononuclear cells in the peripheral blood. Therefore, we hematologically examined monocytes in patients with erythrocytosis. The medical records of 48 erythrocytosis patients (hemoglobin: men, >18.5 g/dL; women, >16.5 g/dL) collected between August 2005 and July 2012 were reviewed for age, gender, hematology, and serum erythropoietin level. Patients were divided into 3 groups: PV (N = 17), reactive erythrocytosis (RE, N = 25), and EPO-elevated erythrocytosis (EPO-E, N = 6). We compared monocytes in the hemogram among the 3 groups and examined the correlation between hematological data and serum EPO level. The percent monocytes in the hemogram were 3.4% ± 0.3%, 7.4% ± 0.4%, and 12.3% ± 1.1% for the PV, RE, and EPO-E groups, respectively, which were significant (p < 0.05). Absolute monocyte counts in the EPO-E group (1.0 ± 0.1 × 10⁹/L) were significantly higher than those in the PV (0.6 ± 0.1 × 10⁹/L) and RE (0.5 ± 0.3 × 10⁹/L) groups. There were 2 (12%), 0, and 3 (50%) patients with monocytosis (>1.0 ′ 10⁹/L) in the PV, RE, and EPO-E groups, respectively. A significant correlation was observed between percent monocyte or absolute monocyte count and serum EPO level. In conclusion, monocyte count in hemograms may be useful for diagnosing EPO-elevated erythrocytosis.