The clinicopathological features of 10 patients with primary mediastinal large B-cell lymphoma (PMLBCL) are described. The patients were aged 19 to 63 years, with a median age of 25.5 years. There were 5 men and 5 women. All patients presented with chest symptoms, and 6 presented with superior vena cava syndrome. Nine patients had bulky mediastinal tumors. The disease was confined within the thorax and contiguous lymph nodes, although multiple liver tumors were observed in 1 patient. Laboratory findings included high lactate dehydrogenase levels and elevated C-reactive protein levels. The soluble interleukin 2-receptor level was high in 6 patients tested. A comparative study of PMLBCL and nodular sclerosis-type 1-Hodgkin's disease (NS-HD) with a mediastinal mass revealed substantial overlap in clinical features. Histopathological examination of biopsy specimens of PMLBCL revealed clusters of CD20(+) large cells; however, CD30(+) Hodgkin/Reed-Sternberg-like cells were occasionally seen, raising the potential to misdiagnose PMLBCL as NS-HD. The patients with PMLBCL were treated with CHOP (cyclophosphamide, doxorubicin, vincristine, and prednisolone), biweekly CHOP, or MACOP-B (methotrexate, doxorubicin, cyclophosphamide, vincristine, prednisolone, and bleomycin) regimen, and 6 received consolidation radiotherapy to the involved field. With the exception of 1 patient who was primarily refractory to therapy, 9 patients (90%) achieved complete response and 7 (70%) remain in continuous remission with a mean follow-up of 24 months. (C) 2001 The Japanese Society of Hematology.