A rare form of congenital adrenal hyperplasia (CAH), 11 beta-hydroxylase deficiency, may be misdiagnosed as 21-hydroxylase deficiency, the most common form of CAH, because of similar clinical presentations at times and elevated level of 17-hydroxyprogesterone in both conditions. We report a case of 11 beta-hydroxylase deficiency that was originally misdiagnosed as 21-hydroxylase deficiency. Hypertension and hypokalemia complicated with seizures and arrhythmia developed in this 9-year-old girl after abrupt withdrawal of oral dexamethasone but maintenance of fludrocortisone. Suspicion of 11 beta-hydroxylase deficiency led to DNA mutation analysis, which revealed a novel point mutation (CTG 461 CCG) in the CYP11B1 gene converting leucine to proline. Her condition stabilized rapidly after withdrawal of fludrocortisone and administration of hydrocortisone. Regular measurement of blood pressure should be performed in all patients with CAH and test of serum 11-deoxycortisol or deoxycorticosterone level should be performed in those patients with elevated blood pressure to avoid misdiagnosis of 11 beta-hydroxylase deficiency.