Introduction: 21-hydroxylase deficiency (21OHD) is the commonest cause of congenital adrenal hyperplasia (CAH), followed in frequency by 11-hydroxylase deficiency (11OHD). Although the relative frequency of 11OHD is reported between 3% and 5% of the cases, these numbers may have been somewhat underestimated. Material and Methods: In 133 patients (89F/44M; 10d-20.9 y) with alleged classic 21OHD and five (3F/2M; 7.3-21 y) with documented 11OHD, we measured serum 21-deoxycortisol (21DF), 17-hydroxyprogesterone (17OHP), and 11-deoxycortisol (S), 48 h after glucocorticoid (GC) withdrawal. We also studied 20 sex- and age-matched control subjects. Serum steroid levels were determined by RIA following HPLC purification. Objectives: 1) quantify 21DF in patients with CAH; 2) correlate hormonal with clinical data; and 3) identify possible misdiagnosed patients with 11 OHD among 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 GC dose. One additional patient with 11OHD was detected (1%) and also one with "apparent combined 11- and 21OHD". S levels were elevated in 11OHD and normal, but significantly higher in 21OHD than in controls. Conclusion: To recognize patients with 21- and/or 11OHD we recommend evaluation of 17OHP or 21DF, and S. Also, 21DF may be useful to follow up pubertal patients with 21OHD. Since 1% of patients with alleged 21OHD may have 11OHD, its frequency seems underestimated, as per our experience in a Brazilian population.