A 38-year-old man was admitted to our department on April, 1991 for a progressive mass in his right thigh. He had pain in the same area since April, 1990, but did not notice lymph node swelling until March, 1991. The biopsy specimen of the mass showed diffuse large cell non-Hodgkin's lymphoma (NHL) with an immunohistochemical feature of B cell. He achieved a complete remission (CR) by a doxorubicin-containing combination regimen and received involved field irradiation with a total dose of 35 Gy thereafter. He has been in CR since September, 1991. Primary NHL of the bone is rare and there have been few reports on MRI findings during the treatment. These findings and management of primary NHL of the bone were discussed.