Background: Early childhood adiposity may have significant later health effects, highlighting the importance of early recognition in young children. This study examines the prevalence and recognition of obesity and severe obesity in 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 2007-2010 within Kaiser Permanente Northern California. Normal BMI (BMI <85th percentile); overweight (BMI 85th-94th percentile); obesity (BMI>=95th percentile); and extreme obesity (BMI>=99th percentile or BMI>=1.2x95th BMI percentile) were classified using the 2000 Center 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% (2,698 of 21,720) of boys and10.0% (2,077 of 20,839) of girls had BMI>=95th percentile. The prevalence was highest among Hispanics (18.2%, 982 of 5,397 boys and 15.2%, 760 of 4,988 girls), followed by blacks (12.4%, 161 of 1,138 boys and 12.7%, 154 of 1,216 girls). A positive trend existed between BMI category and median height percentile, with obesity rates highest in the highest height quintile. The proportion with BMI>=99th percentile was 3.9% (1,670 of 42,559), nearly two-fold higher for boys (66.8%, 1,116 of 1,670) versus girls (33.2%, 554 of 1,670), and identified a larger proportion of children compared to BMI>=1.2x 95th BMI percentile (1.6%, p<0.001). Among those aged 5 years, 77.9% of obese children (1082 of 1389) had provider diagnosis of obesity or elevated BMI, increasing to 84.5% (424 of 502) among 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.