The objectives of this study were (1) to determine the relationship of body mass index (BMI) to primary or secondary hypertension in children and adolescents and (2) to assess BMI at the age of onset of hypertension in children and adolescents. Patient demographics, BMI, family history, presentation of disease, etiology of hypertension, medication, laboratory data, and findings from other procedures were recorded for all patients with hypertension followed in the Pediatric Nephrology Clinic at Children's Hospital, Columbus, Ohio, over a 4-year period. In total, 314 patients were studied: 218 with primary hypertension and 96 with secondary hypertension. Our patient population (166 males, 148 females) was diverse in age (13+/-6.3 years) and ethnicity (237 Caucasians, 54 African-Americans, 23 other). BMI was greater in patients with primary (27.5+/-9.2 kg/m2) versus secondary (23.9+/-9.3 kg/m2) hypertension (P=0.002). Children with primary hypertension with an increased BMI presented at an earlier age than children with secondary hypertension and an increased BMI. The age of onset (10.5+/-2.6 years) in primary hypertension was related to increased BMI (r=0.12, P=0.001); however, there was no relationship between BMI and age of onset of secondary hypertension (P=0.21). Children whose family members had essential hypertension had increased BMI compared with children without a family history of essential hypertension. Based on the logistic regression model constructed from our data, the likelihood of primary versus secondary hypertension was influenced by the presence of family history of hypertension independent of presence of obesity in the child. In conclusion, increased BMI is more common in children with primary than secondary hypertension; earlier onset of primary hypertension in the pediatric population was associated with increased BMI; the assessment of BMI is important in the evaluation of secondary as well as primary hypertension; the role of obesity in the development of secondary as well as primary hypertension in children merits further study.