Androgen deprivation is a cornerstone in prostate cancer management. We present a 69-year-old man, with a poorly differentiated prostate cancer with skeletal and lymph node metastases. After medical and subsequent surgical castration serum testosterone concentrations remained inappropriately high (4.9 and 4.5 nmol/L; castration range <0.5). For cancer staging a CT was performed which showed bilateral adrenal enlargement. Endocrine workup revealed elevated levels of adrenal androgens and adrenal precursors. Mutation analysis confirmed a non-classical 21-hydroxylase deficiency, that is, a mild form of congenital adrenal hyperplasia (CAH). To suppress adrenocorticotrophic hormone and the excess adrenal androgen secretion, treatment with hydrocortisone and prednisolone was started with success. Inadequate testosterone suppression after castration due to previously undiagnosed CAH has not previously been reported. Considering the estimated prevalence of 1% in selected populations, non-classical CAH should be considered when testosterone is not adequately suppressed after castration in men with prostate cancer.