OBJECTIVE: To examine the relationship of body mass index (BMI) and asthma indicators on children with asthma in a Head Start (HS) program. METHODS: In this cross-sectional study (November 18, 2000, to December 12, 2003) of children aged 3 to 5 years with asthma, we compared the BMI data of HS asthmatic patients (n = 213) with the data of peer control subjects from a sample (n = 816) of the National Health and Nutrition Examination Survey aged 3 to 5 years and with children in prekindergarten in Arkansas public schools (n = 1,024). Parental reports of asthma symptoms, health care use, medication use, school days missed, and quality of life were used as indicators of asthma morbidity. Categorical analysis and chi2 tests were performed to examine the relationship between BMI and asthma morbidity. RESULTS: The prevalence of overweight (> or =95th percentile) was significantly higher in HS children with asthma compared with the National Health and Nutrition Examination Survey children (P < .001) and Arkansas prekindergarten children (P = .05). Compared with HS asthmatic children with a BMI less than the 85th percentile, HS asthmatic patients with a BMI of the 85th percentile or greater reported significantly more school days missed (P = .02), lifetime hospitalizations (P = .04), emergency department visits (P = .02), and activity limitations (P = .03) and fewer oral corticosteroid bursts (P = .04). There was also a trend for more daytime symptoms (P = .05) and lower quality of life (P = .06). No differences were observed in rescue (P = .28) or controller (P = .47) medications, environmental tobacco smoke exposure (P = .47), positive allergy test results (P = .85), and nighttime symptoms (P > .99). CONCLUSIONS: Having an increased BMI was associated with more asthma morbidity in this group of HS asthmatic patients. Despite the lack of a clear explanation for the link between asthma and BMI, our data suggest that an increased BMI significantly affects the well-being of young asthmatic patients and should be further addressed.