Objective: To test the validity of four different types of lateral cephalometric radiograph (LCR) measurements as a diagnostic test of adenoid hypertrophy in different age groups of mouthbreathing children. Materials and Methods: Eighty-six mouth-breathing children (male 54.65%, mean age 7.0 6 2.2 years) were randomly selected from a hospital population. Adenoid obstruction of the nasopharynx was evaluated by subjective, linear, ratio, and area LCR measurements. Each measurement was compared with flexible fiberoptic endoscopy diagnosis. Results: Kendall correlation coefficients for agreement between tests were $ 0.67 and kappa scores were substantial ($ 0.64). Higher correlation coefficients and agreement values were found in older age groups. When the sample was stratified by age, the 3-to 5-year-old age group showed lower correlation coefficients and agreement strength for subjective, linear, and ratio measurements. The sensitivity of LCR varied from 71% (ratio) to 84% (linear). The specificity varied from 83% (linear) to 97% (ratio). The positive predictive value varied from 88% (linear) to 97% (ratio). The negative predictive value varied from 70% (ratio) to 78% (linear). The validity of each measure was different among the age groups. Conclusions: LCR is a valid method for measuring adenoid hypertrophy in children from 6 to 12 years old. The diagnosis of adenoid hypertrophy, based on LCR measurements, in children with primary dentition (3-5 years old) should be made with caution. The combination of linear and ratio LCR measurements is a reliable screening tool to determine the need for an ear, nose, and throat evaluation. (Angle Orthod. 2012;82:1001-1007