Here, we report improvement of semen quality in a 30-year-old man with congenital adrenal hyperplasia (CAH) because of 21-hydroxylase deficiency, bilateral testicular adrenal rest tumours (TART) and a 1.5-year infertility history. His adrenal substitution therapy was changed from hydrocortisone 10 mg once daily to 10 mg three times daily in combination with dexamethasone, 0.1 mg once daily. Upon this change, the testicular tumours showed regression and his sperm concentration increased from 0.1 to 98 mio/mL and total sperm count from 0.64 to 392 mio. sp., total number of motile spermatozoa increased from 0.23 to 258.72 mio. The total number of morphologically normal spermatozoa increased from 0.01 to 19.6 mio. sp. The patient reported improved well being and did not develop signs of overtreatment. A non-optimal medical substitution therapy of this CAH patient is the most likely explanation for the presence of TART and disturbed reproductive hormones levels, leading to impaired semen quality. Optimizing the medical treatment may at least in some cases improve fecundity.