A 55-year-old woman underwent breast-conserving surgery with irradiation for Stage IIB (T2 N1 M0) breast cancer of her right breast. Thereafter, she was treated orally with oral UFT and tamoxifen. Three years following surgery, she was diagnosed as having a non-Hodgkin's lymphoma. She underwent 6 cycles of EPOCH-G (etoposide, vincristine, adriamycin, cyclophosphamide, prednisolone, G-CSF) therapy, and obtained complete remission. Two years later, her neck and inguinal lymph nodes were swollen. Biopsy confirmed the relapse of NHL. She underwent salvage chemotherapies of MST-16, carboplatin, IVAC, and CPT-11, but the disease was refractory. In May 2000, bilateral pleural effusion was detected. Cytomorphologically, the pleural fluid specimen showed both atypical lymphoid cells and adenocarcinoma cells simultaneously. Existence of double cancer in the pleural effusion has not been reported, suggesting that this case is rare.