Introduction: 21-Hydroxylase deficiency (21OHD) is the most common cause of congenital adrenal hyperplasia, followed in frequency by 11 beta-hydroxylase deficiency (11 beta OHD). Although the relative frequency of 11 beta OHDis reported as between 3 and 5% of the cases, these numbers may have been somewhat underestimated. Materials and Methods: In 133 patients (89 females/44 males; 10 d-20.9 yr) with alleged classic 21OHD and five ( three females/two males; 7.3-21 yr) with documented 11 beta OHD, we measured serum 21-deoxycortisol (21DF), 17-hydroxyprogesterone (17OHP), and 11-deoxycortisol ( S), 48 h after glucocorticoid withdrawal. We also studied 20 sex- and age-matched control subjects. Serum steroid levels were determined by RIA after HPLC purification. Objectives: The objectives of this study were to: 1) quantify 21DF in patients with congenital adrenal hyperplasia, 2) correlate hormonal with clinical data, and 3) identify possible misdiagnosed patients with 11 beta OHD among those with 21OHD. Results: In 21OHD, 17OHP (217-100,472 ng/dl) and 21DF (< 39-14,105 ng/dl) were mostly elevated and positively correlated ( r = 0.7202; P < 0.001). Except for higher 17OHP in pubertal patients, 17OHP and 21DF values were similar according to sex, disease severity, or prevailing glucocorticoid dose. One additional patient with 11 beta OHDwas detected (1%) and also one with apparent combined 11 beta-and 21OHD. S levels were elevated in 11 beta OHD and normal but significantly higher in 21OHD than in controls. Conclusion: To recognize patients with 21- and/or 11 beta OHD, we recommend evaluation of 17OHP or 21DF and S. Also, 21DF may be useful to follow up pubertal patients with 21OHD. Because 1% of patients with alleged 21OHD may have 11 beta OHD,its frequency seems underestimated, as per our experience in a Brazilian population.O TEXTO COMPLETO DESTE ARTIGO, ESTARÁ DISPONÍVEL À PARTIR DE FEVEREIRO DE 2015.91621792184