OBJECTIVE To examine trends in attention-deficit/hyperactivity disorder (ADHD) by race/ethnicity, age, sex, and median household income. DESIGN An ecologic study of trends in the diagnosis of ADHD using the Kaiser Permanente Southern California (KPSC) health plan medical records. Rates of ADHD diagnosis were derived using Poisson regression analyses after adjustments for potential confounders. SETTING Kaiser Permanente Southern California, Pasadena. PARTICIPANTS All children who received care at the KPSC from January 1, 2001, through December 31, 2010 (n = 842 830). MAIN EXPOSURE Period of ADHD diagnosis (in years). MAIN OUTCOME MEASURES Incidence of physician-diagnosed ADHD in children aged 5 to 11 years. RESULTS Rates of ADHD diagnosis were 2.5% in 2001 and 3.1% in 2010, a relative increase of 24%. From 2001 to 2010, the rate increased among whites (4.7%-5.6%; relative risk [RR] = 1.3; 95% CI, 1.2-1.4), blacks (2.6%- 4.1%; RR = 1.7; 95% CI, 1.5-1.9), and Hispanics (1.7%-2.5%; RR = 1.6; 95% CI, 1.5-1.7). Rates for Asian/Pacific Islander and other racial groups remained unchanged over time. The increase in ADHD diagnosis among blacks was largely driven by an increase in females (RR = 1.9; 95% CI, 1.5-2.3). Although boys were more likely to be diagnosed as having ADHD than girls, results suggest the sex gap for blacks may be closing over time. Children living in high-income households were at increased risk of diagnosis. CONCLUSIONS The findings suggest that the rate of ADHD diagnosis among children in the health plan notably has increased over time. We observed disproportionately high ADHD diagnosis rates among white children and notable increases among black girls.