Classic congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency is characterized by decreased synthesis of glucocorticoids and mineralocorticoids, adrenal hyperandrogenism, and impaired development and function of the adrenal medulla. Although genotype can usually predict phenotype, genotype-phenotype discordance has been described. We investigated the association between adrenomedullary function, disease severity, and genotype in 37 children [22 males and 15 females; age range, 4.7-14.9 yr; 28 salt-wasting (SW) and 9 simple virilizing (SV) CAH] with classic 21-hydroxylase deficiency. Plasma and 24-h urinary catecholamines and their metabolites, and the 21-hydroxylase genotype were determined in all patients. The disease-causing mutations were divided into 4 groups (Null, A, B, and C) according to in vitro 21-hydroxylase activity as previously described. Genotype groups Null (n = 9) and A (n = 15) were predicted to result in SW CAH, group B (n = 8) was predicted to have the SV phenotype, and group C (n = 1) was predicted to have nonclassic CAH. A fifth group, D (n = 4), included patients in whom mutations were detected in only 1 allele. Plasma total metanephrine (420.1 +/- 60.0 vs. 657.7 +/- 67.8 pg/ml; P = 0.04) and free metanephrine (13.4 +/- 1.7 vs. 24.0 +/- 4.1 pg/ml; P = 0.008) concentrations were significantly lower in children with SW CAH than in those with the SV form of the disease. Plasma free metanephrine concentrations best predicted phenotype, with accuracy similar to that of genotype. Concordance rates between genotype and phenotype were higher in the most severely affected patients (Null, 88.9%; A, 93.3%; B, 75%; plasma free metanephrine, <18.5 pg/ml: SW, 92%). The plasma free metanephrine concentration correlated with the expected 21-hydroxylase activity based on genotype, and there was a significant trend for free metanephrine concentrations across the three genotype groups (P < 0.0001). Our findings indicate that measurement of adrenomedullary function, best assessed by the free metanephrine concentration, is a useful biomarker of disease severity in 21-hydroxylase deficiency. Molecular genotype and plasma free metanephrine concentration predict phenotype with similar accuracy. Both methods are more accurate in the most severe forms of the disease.