This study was performed to determine whether hematologic indexes might show heterogeneity among different subgroups of the population and, if differences were found, to develop reference ranges for each subgroup. Complete blood cell counts and leukocyte differential cell counts were compared in healthy white (n = 663), black (n = 697), Latin-American (n = 535), and Asian (n = 247) adults. Women had significantly lower red blood cell counts, hemoglobin levels, and hematocrits than men. In addition, women in at least three racial/ethnic groups, had higher platelet, white blood cell, and granulocyte counts and lower monocyte counts than men. Among blacks and Latin Americans, women had lower mean corpuscular volumes, mean corpuscular hemoglobin levels, and mean corpuscular hemoglobin concentrations than men. The following racial/ethnic differences were found: higher red blood cell distribution widths and lymphocyte and monocyte counts, and lower hemoglobin levels, hematocrits, mean corpuscular hemoglobin levels, mean corpuscular hemoglobin concentrations, and white blood cell, granulocyte, and band counts among blacks; higher white blood cell, granulocyte, and band counts among Latin Americans; and higher red blood cell and lower lymphocyte and monocyte counts among Asians than one or more of the other three groups. We concluded that the complete blood cell counts and leukocyte differential cell counts exhibit marked heterogeneity and that reference ranges that reflect the proper racial, ethnic, and sex group should be developed for these parameters.