Nonalcoholic fatty liver disease (NAFLD) is common in overweight adolescents, and screening with serum alanine transaminase (ALT) levels is recommended by the American Academy of Pediatrics. We sought to determine if ALT is an accurate adolescent screening measure for NAFLD in a nationally representative sample of overweight adolescents. Diagnosis of NAFLD was determined using vibration-controlled transient elastography. Analyses were performed to calculate the sensitivity, specificity, positive predictive value, negative predictive value, and Youden’s index at various serum ALT cutoff levels. Receiver operating curves were generated in order to determine ALT’s discrimination capability. Males and females were analyzed separately. While average measures (mean and median) of ALT were higher in subjects with NAFLD, ALT provided only minimal discrimination with AUROC (area under the receiver operating characteristic) values of .66 in males and .67 in females. In a nationally representative sample of overweight and obese adolescents, serum ALT level functioned inadequately as a screening test to detect NAFLD.