['The effectiveness of biological investigations aiming at discriminating isolated hypogonadotropic hypogonadism (IHH) from constitutional delayed puberty (CDP) in male patients is still controversial. We revisited the diagnostic power of the basal serum testosterone level, the Triptorelin test and the human chorionic gonadotropin (hCG) test in a cohort of 33 boys with delayed puberty.', 'Boys were aged 14.2 to 26.2 Years at referral. A 5-Year-long clinical follow-up after the initial study allowed confirmation of the diagnosis. At the end of the follow-up period, IHH was found in 13 patients while the other 20 had normal spontaneous pubertal development (CDP).', 'At referral, a basal morning testosterone level >1.7 nmol/l was observed in 55% of patients with CDP exclusively (predictive positive value (PPV)=100%; predictive negative value (PNV)=59%). For CDP, the PPV of the LH peak 3 h after Triptorelin was 100% by setting the upper threshold at 14 IU/l and the PNV was 72%. However, no lower threshold could discriminate IHH from CDP in the remaining patients with an LH peak 3 h after Triptorelin <14 IU/l. In CDP patients, the PPV of the serum testosterone increment after hCG stimulation (deltaT/hCG) was 100% for values >9 nmol/l (PNV=72%). In IHH patients, the PPV of deltaT/hCG was 100% for values <3 nmol/l (PNV=82%). Only 29% of the studied population had a deltaT/hCG between these lower and upper thresholds and therefore could not have been classified initially.'] Is the human chorionic gonadotropin test more powerful than the gonadotropin-releasing hormone agonist test to discriminate male isolated hypogonadotropic hypogonadism from constitutional delayed puberty?