21-Hydroxylase deficiency (21OHD) is the commonest form of congenital adrenal hyperplasia, while 11βOHD represents 5% of cases. Although both result from mutations in distinct genes, cases of 'apparent' combined 21OHD and 11βOHD (AC21,11OHD) have been occasionally reported. A 6 year-old girl, born with ambiguous genitalia and salt-loss, had serum elevations (ng/dl) of androstenedione (>1,000), 17-hydroxy-progesterone (170HP; 38,483), 21-deoxycortisol (21DF; 23,338), and 11-deoxycortisol (S; 4,928), suggesting AC21,11OHD. CYP21A and CYP11B1 genotyping identified mutations only in the former. On follow-up, serum S became normal but 170HP and 21DF were still elevated. ACTH stimulation disclosed elevated levels of 170HP and 21DF, but unresponsive S and undetectable deoxycorticosterone. The hormonal pattern initially suggested AC21,11OHD), but subsequent normalization of S showed transient 11-hydroxylase inhibition. 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