A patient with primary malignant lymphoma of the urinary bladder is presented. Grossly, the bladder showed multiple submucosal masses. Histologically and immunohistochemically, diffuse B-cell lymphoma of the medium-sized cell type was revealed. On the basis of clinicopathological features, the present case resembled previously recorded cases of bladder lymphoma. The pathogenesis of this primary bladder lymphoma was presumably associated with follicular or chronic cystitis. Primary lymphoma of the bladder is a condition that is very rarely included in series of extranodal lymphomas, and there is a curious sex difference in its occurrence rates between Japan and Western countries. Primary lymphoma of the bladder may be considered a lymphoma that originates fro mucosa-associated lymphoid tissue.