A report that elevated urinary lactic acid dehydrogenase (LDH) isoenzyme 5 activity is a reliable tool for separating patients with upper from those with lower urinary tract infections (UTIs) led us to study urinary LDH enzyme activity in girls having bladder washout studies to localize the site of infection. Urinary LDH isoenzyme 5 activity in 64 instances of lower UTI was 16.1 +/- 3.3%, a value not significantly different than that of 18.2 +/- 12.6% found in 26 instances of upper tract infection (t = 0.8726, P = 0.1928). The data show that LDH isoenzyme 5 activity is of no value for localization of the site of a UTI. The data of these studies also showed that urinary LDH enzyme activity clearly separates girls with UTIs from those without infections, but it is unlikely that this finding will be of value in diagnosis or management.