A 20 year old man otherwise fit and well, presented with a seizure, fall and fractured neck of femur. DEXA scan revealed osteoporosis; baseline blood tests were apparently normal in the endocrinology clinic with a Testosterone level of 24.3 nmol/l (Range 9.927.8) and normal bone and thyroid profile. He was then lost to follow-up. 4 years on, he was re-referred to the endocrinology clinic with a surprisingly elevated testosterone level of 34.6 nmol/l in combination with osteoporosis, Further investigations showed FSH 64.2 IU/l (Range 1.012.0) and LH 22.6 IU/l (Range 0.612.1) to be raised. He was noted to be of medium build with a male pattern hair distribution and full beard, normal penile length with testes size of borderline volume of 15 ml. He was not in a relationship but claimed to have had normal sexual development on par with his peers, SHBG came back as very high at 142 nmol/l (Range 1471); bioavailable testosterone was calculated to be low at 5.32 nmol/l. Genetic testing revealed Klinefelters Syndrome with mosaicism of 46XY/ 47XXY. Topical testosterone resulted in significant improvement in general wellbeing. Mosaicism is seen in 10% of Klinefelters cases and is less often diagnosed due to the milder phenotype as seen in our patient; a high index of suspicion is needed for diagnosis. SHBG is often elevated and cannot be explained simply by the elevated oestradiol in Klinefelters. Diagnosis was complicated in this case by the patient being on first generation of antiepileptics also contributing to the SHBG elevation. Although the most common cause of primary hypogonadism with an incidence of 1 in 750, 75% of cases of Klinefelters are never diagnosed and many are diagnosed later in life. Delay in diagnosis often leads to irreversible physical, mental and social health consequences.