Urinary lactic dehydrogenase (LDH) activity and isozyme pattern were investigated in 38 patients with bladder tumors of various stages and grades and in 10 healthy persons. Urinary LDH activity was markedly elevated and directly correlated with histologic stage and grade of tumors. The urinary LDH isozyme pattern differed significantly from that of normal urine even in early and well-differentiated tumor cases. LDH4 and LDH5 became more prominent as the degree of the histologic stage and grade of the tumor increased. In the patients with tumors more advanced than stage T2, LDH4 and LDH5 were greater than LDH1 plus LDH2, the reverse of what is found in normal urine. As analogous result was obtained in patients with tumors of grade II or higher. No appreciable difference was found in the LDH isozyme pattern of patients with single or multiple tumors although the total activity was greater with multiple tumors. A comparison between LDH isozyme pattern in urine and in tumor tissue was evaluated in 26 patients with a good correlation (r = 0.40, P less than 0.05). Comparison of the results with urine LDH and urine cytology was attempted for each stage and grade. In 5 of 16 cases having negative urine cytology, a positive isozyme index was present and 10 of 13 cases having positive urine cytology had a positive isozyme index. These data suggest that evaluation of urinary LDH combined with urinary cytology might be of value in the diagnosis of bladder tumors and for following patients who have had bladder tumors.