Blood group antigens (BGA) were stained by PAP methods and the relation between staining pattern (classified into five types: I, II, IIIa, IIIb and IV) and tumor prognosis was examined in 54 patients with primary superficial bladder tumor. The recurrence rate was significantly higher in BGA negative (Type IIIb or IV) or Type IIIa cases than in antigen positive (Type I or II) cases. Stage-up was observed only in BGA negative cases. Although staining pattern of recurrent tumor differed from that of primary tumor in some cases, it is believed that prediction of malignant potential should be performed at the primary development, when tumor has not been influenced by treatments. With regard to the relation between staining pattern and tumor prognosis, no significant difference was found between the cases of A, B and AB blood groups and those of O group.