Gynecomastia is a benign enlargement of the male breast attributable to proliferation of the ductal elements. Gynecomastia has been rarely reported as an adverse effect of isoniazid therapy. We report the case of a 35-year-old man who was prescribed with isoniazid, rifampicin, ethambutol and pyrazinamide to treat pulmonary and lymphatic tuberculosis. After five months of treatment, the patient complained of painful engorgement in the bilateral breasts and the presence of male gynecomastia was confirmed with a physical examination and radiographical methods. The serum level of estradiol was also increased. Common causes of male gynecomastia were excluded through history taking and the laboratory findings. The anti-TB drugs were changed to a second line regimen due to radiographical progression and the intolerance of the patient to gynecomastia. Gynecomastia was relieved very slowly and a tender subareolar palpable mass decreased in size and consistency over five-month period after stopping the probable causative drug, isoniazid. From a review of the literature, gynecomastia has been shown to be a side effect of treatment with first line anti-tuberculosis drugs, and especially with isoniazid. We report the rare case.