Macronutrient modification may play a role in normalizing the endocrine-metabolic state characteristically perturbed during the pubertal transition. This study evaluated the effect of a reduced carbohydrate (redCHO) diet relative to a standard (STAN) diet on the reproductive hormonal milieu and insulin dynamics in obese African American (AA) girls during the pubertal transition. The authors hypothesized that a 5-week eucaloric, redCHO diet would decrease fasting insulin and estradiol (thereby altering follicle stimulating hormone [FSH] and sex-hormone-binding globulin [SHBG] concentration) and improve insulin sensitivity, relative to a STAN diet. Participants were assigned to 1 of 2 diets: redCHO (43% carbohydrate) or STAN (55% carbohydrate) for 5 weeks. All food was provided with energy needs determined by resting energy expenditure multiplied by a 1.2 activity factor. This relatively short, redCHO dietary intervention affected reproductive hormonal concentrations but not insulin dynamics. An inverse relationship between dietary carbohydrate and FSH (P = .04) was observed as was a marginal positive relationship between dietary carbohydrate and SHBG (P = .06). When the sample was stratified by baseline median fasting insulin (<14.0 mg/dL), an inverse albeit nonsignificant association between the redCHO diet and estradiol (P = .10) was observed only in those with higher insulin. Macronutrient modification has the potential to affect the reproductive hormonal milieu, possibly via effects on insulin dynamics. Additional therapies targeting insulin homeostasis and/or fuel oxidation in conjunction with dietary manipulation may be able to improve hormonal, metabolic, and thus body composition outcomes. © 2011, SAGE Publications. All rights reserved.