The prevalence of neutropenia (neutrophil count <1.5 × 109 cells/L) differed by age, sex, and ethnicity (Figure 2; Appendix Table 4). A total of 583 participants had neutrophil counts less than 1.5 × 109 cells/L. The weighted prevalence was 1.2% (95% CI, 1.1% to 1.4%), which represented an estimated 3.1 million persons in the United States. Neutropenia was present in 4.5% (CI, 3.9% to 5.0%) of black participants, 0.79% (CI, 0.57% to 1.0%) of white participants, and 0.38% (CI, 0.24% to 0.52%) of Mexican-American participants. Across ethnic groups, males were more likely to have neutropenia: 6.65% for black males versus 3.57% for black females, 0.90% for white males versus 0.59% for white females, and 0.57% for Mexican-American males versus 0.39% for Mexican-American females. The prevalence of neutropenia was highest in children younger than age 5 years, was generally lower during childhood, and reached the adult level by age 15 to 17 years in females and by age 18 to 24 years in males. We collapsed the 25- to 64-year-old age groups because the prevalence of neutropenia showed no trend in this broad age range (data not shown). For every age and sex category, black participants were more likely than white participants to have neutropenia; in most age and sex categories, neutropenia was less common among Mexican-American participants than among white participants.