3 beta-Hydroxysteroid dehydrogenase type II deficiency (3 beta HSD2) congenital adrenal hyperplasia is a rare cause of ambiguous genitalia, resulting in abnormal virilisation in both 46XY and 46XX. We describe a case of 46XY ambiguous genitalia that was misdiagnosed as androgen insensitivity syndrome. The correct diagnosis was made after adrenarche. Genotyping demonstrated compound heterozygosity in two alleles, the previously described c.244G >A (p.Ala82Thr), and a novel 931C >.T(p. Gln311*) variant. We suggest that adrenarche unmasked the condition by driving cortisol production to rates that caused the mutant 3bHSD2 enzyme to become rate limiting for cortisol production. This case illustrates how markedly different the effects of this condition may be on androgen production compared with glucocorticoid and mineralocorticoid production. It also demonstrates how current guidelines based on urinary steroids and cortisol sufficiency may not arrive at the correct diagnosis, and underlines the importance of gene testing in the work-up of disorders of sexual differentiation.