WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT: The prevalence of obesity in the United States has increased dramatically over the past three decades. There is a growing spectrum of severe obesity among children and adolescents. Obesity trends and race/ethnic differences may be evident at a young age. WHAT THIS STUDY ADDS: Among children aged 3-5 years, the prevalence of obesity and severe obesity was higher in boys than in girls, and highest among children of Hispanic ethnicity. Within this young age group, higher body mass index (BMI) was associated with greater height percentile. Among obese children aged 5 years, provider recognition of obesity or elevated BMI was high, approaching 80% of children. BACKGROUND: Early childhood adiposity may have significant later health effects. This study examines the prevalence and recognition of obesity and severe obesity among preschool-aged children. METHODS: The electronic medical record was used to examine body mass index (BMI), height, sex and race/ethnicity in 42 559 children aged 3-5 years between 2007 and 2010. Normal or underweight (BMI < 85th percentile); overweight (BMI 85th-94th percentile); obesity (BMI >/= 95th percentile); and severe obesity (BMI >/= 1.2 x 95th percentile) were classified using the 2000 Centers for Disease Control and Prevention growth charts. Provider recognition of elevated BMI was examined for obese children aged 5 years. RESULTS: Among 42 559 children, 12.4% of boys and 10.0% of girls had BMI >/= 95th percentile. The prevalence was highest among Hispanics (18.2% boys, 15.2% girls), followed by blacks (12.4% boys, 12.7% girls). A positive trend existed between increasing BMI category and median height percentile, with obesity rates highest in the highest height quintile. The prevalence of severe obesity was 1.6% overall and somewhat higher for boys compared with girls (1.9 vs. 1.4%, P < 0.01). By race/ethnicity, the highest prevalence of severe obesity was seen in Hispanic boys (3.3%). Among those aged 5 years, 77.9% of obese children had provider diagnosis of obesity or elevated BMI, increasing to 89.0% for the subset with severe obesity. CONCLUSIONS: Obesity and severe obesity are evident as early as age 3-5 years, with race/ethnic trends similar to older children. This study underscores the need for continued recognition and contextualization of early childhood obesity in order to develop effective strategies for early weight management.