Many patients with ileal conduit urinary diversion have infected urine but far fewer have clinical pyelonephritis. A noninvasive diagnostic test to distinguish renal bacteriuria from conduit colonization in these patients would seem desirable. Urine total lactic dehydrogenase and lactic dehydrogenase isoenzymes, and serum C-reactive protein have been useful to distinguish pyelonephritis from cystitis in patients with intact urinary tracts. We used these tests in patients with ileal conduits who had urine containing more and less than 10(5) organisms per ml. All patients had elevated urine total lactic dehydrogenase-5 isoenzyme, and serum C-reactive protein. No statistically significant difference in any of these parameters existed between the groups. These results may indicate that all patients with conduits have pyelonephritis but only intermittently demonstrate bacteriuria, or that the conduit mucosa contributes lactic dehydrogenase to the urine. However, it does not appear that these tests alone can distinguish accurately renal bacteriuria from conduit colonization.