Klinefelter syndrome (KS) can involve various psychiatric symptoms that are often heterogeneous and atypical. To illustrate the diagnosis and treatment difficulties of psychiatric non-specific symptoms occurring in KS, we report a 17-year-old man presenting with gynaecomastia and marfanoid features. Investigations showed high concentrations of gonadotrophins and a 47,XXY karyotype. Although his serum testosterone was normal, the patient had clinical hypogonadism and suffered from recent headache. Magnetic resonance imaging revealed pituitary non-invasive macroadenoma. Treatment with Cabergolin resulted in a normalisation of prolactin levels and a decrease in tumour size. During follow-up, the patient presented a permanent state of passivity and disinterest in imaginative experiences, without any obvious specific diagnoses, according to psychiatric examination. Further investigations showed major depression, schizotypical personality and patterns of psychotic functioning. Initial treatment with selective serotonin re-uptake inhibitors was not effective and he was switched to an atypical antipsychotic drug that was not tolerated and rapidly stopped by the patient.