Klinefelter syndrome (47, XXY in most cases) is a frequently underdiagnosed chromosomal anomaly associated with multiple comorbidities in adult life. Patients with Klinefelter syndrome have a higher risk of cancer. Specifically, these patients have a higher risk for mediastinal germ cell tumors. It is estimated that 8% of male patients with mediastinal tumors have Klinefelter. We report a 42-years-old male who suffered recurrent respiratory infections. During the study, a mediastinal mass was found, whose pathological study disclosed a type B thymoma. The patient had a history of infertility, high stature, gynecomastia, obesity with gynecoid distribution of body fat and testicular atrophy. A karyotype was requested (47, XXY), confirming the diagnosis of Klinefelter syndrome.