Left ventricular hypertrophy has been documented in hypertensive adolescents and among some with prehypertension. Obesity also appears to be associated with cardiac mass, independent of blood pressure (BP). Fibroblast growth factor 23 (FGF23) is a novel biomarker positively associated with left ventricular hypertrophy in adults with and without kidney disease. The aim of this study was to determine if there was a significant and independent association of FGF23 with cardiac mass in a Black American adolescent cohort including both normotensive and prehypertensive participants with and without obesity. Measurements of BP, body mass index (BMI), plasma c-terminal FGF23, and echocardiographic measures of left ventricular mass index (LVMI) were obtained in 236 adolescents, aged 13-18 years, stratified by BMI as normal, overweight, or obese. LVMI differed significantly between normal, overweight, and obese groups (30.42 +/- 6.75 vs. 33.49 +/- 8.65 vs. 37.26 +/- 6.99 gm/m2.7; P < .01). FGF23 was significantly higher in both overweight (53.03 RU/mL) and obese (54.40 RU/mL) compared to the normal weight (32.83 RU/mL) group (both P < .01). In multiple linear regression analysis, variables significantly related to LVMI in males were BMI (P < .0001) and FGF23 (P = .005), but not BP, high-sensitivity C-reactive protein, or insulin. The only significant variable associated with LVMI in females was BMI (P < .0001). In males, the contribution of FGF23 to predicting LVMI was independent of and in addition to obesity. These results suggest that FGF23 is an integral part of a complex pathway, associated with higher cardiac mass in African-Americans males with excess adiposity.