A patient with widespread skin lesions presenting with massive pleural effusion.

Mycosis fungoides is the most commonly seen type of cutaneous T-cell lymphoproliferative disease. While mycosis fungoides is linked to an increased risk of developing secondary malignancies, the occurrence of B-cell-originated disease in association with it is exceedingly rare. A 66-year-old male with persistent papillomatous skin eruption was admitted due to dyspnea. Chest X-ray, positron emission tomography, and chest computed tomography revealed axillary and mediastinal lymph node enlargement and right lower pulmonary lobe infiltration along with right-sided massive pleural effusion. Histological and immunohistochemical findings of pleural biopsy and axillary lymph nodes suggested a diagnosis of pulmonary extranodal marginal zone lymphoma. Skin biopsies from the abdomen, chest, and legs revealed CD4/CD8 double-positive patch stage of mycosis fungoides. After completing six cycles of chemotherapy, complete remission of lymphoma was achieved, with the skin eruptions remaining unchanged. Herein, the authors present a unique case of concomitant diagnoses of mycosis fungoides and marginal zone B-cell lymphoma of the respiratory system to emphasize the importance of careful evaluation of each finding.