Approximately 20% of children with idiopathic slipped capital femoral epiphysis (SCFE) have bilateral disease. Predicting which patients will develop problems with both hips remains difficult. This is the first study to evaluate the relationship between body mass index (BMI) and unilateral and bilateral SCFEs. Height and weight measurements of patients presenting with SCFE to our institution were obtained and used to calculate the BMI. Of the 54 patients enrolled in the study, 16 ultimately had bilateral disease. The mean BMI of patients with bilateral disease was significantly greater than that of patients with unilateral disease. In addition, patients presenting with unilateral involvement who progressed to bilateral disease had a significantly greater average BMI than patients who did not progress. Elevated BMI is associated with SCFE, especially bilateral SCFE.