21-hydroxylase deficient - congenital adrenal hyperplasiais a disease caused by a mutation in the CYP21A2 gene, resulting loss of function of the 21-hydroxylase enzyme, a key enzyme involved in the synthesis of cortisol, resulting in reduced cortisol synthesis, increased testosterone synthesis which lead to the clinical manifestations of acute adrenal failure, virilizing in girls and early puberty in boys. The disease vary in term of clinical manifestations and is classified into three major disease categories: salt wasting, simple virilizing, and non-classical. Classification of disease and clinical characteristics in patients with congenital adrenal hyperplasia is important in the diagnosis and treatment of the disease. Objective: To determine the incidence and clinical characteristics of patients with congenital hyperplasia due to 21-hydroxylase deficiency. 209 patients admited to the National Children Hospital were analyzed and evaluated clinically. Results showed that 77% of patients were classified as salt wasting, 21% as simple virilizing and 2% as non-classical. Patients in the salt wasting and non-classical group were diagnosed earlier (32 days and 18.5 days) than those in the simple virilizing group (1590 days) (p<0.001). The proportion of males/females were nearly equal in salt wasting group (46.6% males and 54.4% females). Simple virilizing group has a higher proportion of female patients (81.8% female and 18.2% male). The incident of gender ambiguity in female patients in all three categories of salt loss, male and female, were similar (96, 97, and 100%, respectively). Acute adrenal failure was found only in patients with salt wasting form with 91.8% in male and 78.6% in female.