Klinefelter's syndrome (KFS) tends to be associated with immunological disorders. We describe a 37-year-old man who presented signs of testicular atrophy and decreased body hair. He showed pancytopenia and elevated levels of liver enzymes. Chromosome analysis revealed 47XXY karyotype; therefore, he was diagnosed with KFS, with systemic lupus erythematosus and autoimmune hepatitis. Treatment with a high dose of methylprednisolone and methyltestosterone improved thrombocytopenia and symptoms, suggesting that methyltestosterone may have a clinical benefit in the treatment of KFS with a low level of testosterone accompanying immunological disorders.