Objectives: To determine whether menarcheal status in individual girls biases assessments of overweight and obesity when using body mass index (BMI) criteria from age-based reference data. Study design: Data were analyzed for 2145 girls aged 8.00 to 16.99 years was examined from 1999 to 2002 as part of the National Health and Nutrition Examination Survey (NHANES). Results: An estimate of the menarche-related effect on BMI assessment at each age was calculated as K = (m-M)β, in which m = the menarcheal status of a girl (0,1); M = the prevalence of menarcheal status; and β = the regression coefficient of BMI on m. At levels of BMI near the percentile cutoff points defining overweight and obesity (85th, 95th) the prevalence of menarcheal status is higher and the BMI differences between menarcheal and premenarcheal girls (β) are smaller than those observed when all girls are considered. The average effects on BMI of menarche that occurs earlier than the population mean age or of menarche that occurs later than the population mean age generally are ≤ ± 1.00 kg/m2 for individual girls whose BMI is near the cutoff values defining overweight and obesity. Conclusion: The average effects of menarcheal status on BMI assessments of overweight and obesity are small and usually should not be clinically important.