This was a rare case, in which clinical manifestation was a thymus area tumor accompanied by proteinuria, kidney dysfunction and kidney enlargement. Tumor biopsy and bone marrow examination failed to clarify its diagnosis. Renal biopsy revealed diffuse infiltra-tion by atypical lymphoid cells in renal interstitium. Immunoperoxidase studies demonstrated this case was non-Hodgkin’s lymphoma(NHL) of the B cell type through the renal biopsy.