The clinical, histological and immunohistochemical findings in five primary lymphomas of the urinary bladder are reported. One patient had both lymphoma and transitional cell carcinoma of the bladder. All of the lymphomas showed histological features of mucosa-associated lymphoid tissue (MALT) lymphomas with centrocyte-like cells in all cases. One patient with pre-existing cystitis glandularis showed lymphoepithelial lesions. Biopsies from four patients contained reactive germinal centres and, in two of these, there was follicular colonization by tumour cells. In three patients, repeat biopsies, over several years, showing the changes of MALT lymphoma, were diagnosed as cystitis. We suggest that a large proportion of primary lymphomas of the bladder are lymphomas of MALT and that the characteristic morphological and immunohistochemical features of these tumours should be sought in biopsies containing large numbers of lymphoid cells.