Background: Whether and to what extent racial/ethnic disparities in attention-deficit/hyperactivity disorder (ADHD) diagnosis occur by kindergarten entry is currently unknown. We investigated risk factors associated with an ADHD diagnosis by kindergarten entry generally, and specifically whether racial/ethnic disparities in ADHD diagnosis occur by this very early time period. Methods: Secondary analysis of data from children enrolled in the Early Childhood Longitudinal Study-Birth Cohort (ECLS-B), a large, nationally representative cohort of US children born in 2001. Data include information from birth certificates, parent and teacher questionnaires, and in-person developmental assessments conducted with children at intervals from 9 months through kindergarten entry. The analytic sample included children enrolled in the ECLS-B at the 60-month assessment (N = 6,550). Results: Black children in the United States were 70% (1 -"OR" of 0.30) less likely to receive an ADHD diagnosis than otherwise similar White children. Hispanic children initially appeared to be underdiagnosed for ADHD. However, their disparity with Whites became statistically nonsignificant after controlling for whether a language other than English was primarily spoken in the home. Analyses of kindergarten teacher-reported classroom behavior indicated that neither Black nor Hispanic children displayed less frequent ADHD-related behaviors than Whites. Conclusions: Although they are not less likely to display ADHD-related behaviors, children who are Black or being raised in households where non-English is primarily spoken are less likely than otherwise similar White children to be diagnosed with ADHD in the US.