<i>Introduction:</i> Gynecomastia is a common clinical sign in several diseases. In this report we present a case of gynecomastia with underlying testicular tumor which remained misdiagnosed for a prolonged period of time. <i>Case Report:</i> A 16-year-old adolescent noticed unilateral painless swelling of the left breast. He was referred to the Department of General Surgery and examined by a breast surgeon. A diagnosis of mastitis was made and a treatment with an oral antibiotic drug began. After failure of the initial antibiotic treatment, the patient was referred to the Department of Endocrinology and left testicular cancer was diagnosed. Unilateral high inguinal orchidectomy and subsequent chemotherapeutic treatment were performed. <i>Conclusion:</i> Primary care physicians should be aware of the possibility of a concomitant presence of gynecomastia and testicular cancer. We suggest a physical examination as well as a laboratory investigation, and testicular ultrasonography of the testes in all patients with gynecomastia.