A number of indirect methods have been developed to determine the site of urinary tract infection, including the measurement of LDH in urine [1]. Although LDH has been thought to be from the kidneys, it has also been noted that leukocytes could contribute LDH isoenzymes 4 and 5 [2]. Seventeen patients with injured spinal cords and significant bacteriuria were included in this study. Urine specimens obtained by urethral catheter were cultured, and PMNLs identified with Sternheimer-Malbin stain were counted in a hemacytometer. A positive test for antibody-coated bacteria and the lack of patient response to five to 10 days of antibiotic therapy were used as an indication of upper urinary tract infection. Levels of LDH isoenzymes 4 and 5 (cathodal) correlated with the number of PMNLs in the urine (r = 0.63, P less than 0.01). There was no correlation of PMNLs with LDH isoenzymes 1 and 2 (r = 0.18). In addition, there was no correlation of LDH isoenzymes 4 and 5 with the level of urinary tract infection. These results suggest that the PMNLs in the urine are the source of the LDH isoenzymes 4 and 5.