The presence of tubular involvement, as a marker for the detection of urinary tract infection (UTI) site, was examined in 19 patients with pyelonephritis and in 15 patients with cystitis or asymptomatic bacteriuria. The urinary excretion of four markers of tubular proteinuria, β<sub>2</sub>-microglobulin (β<sub>2</sub>M), lysozyme (LZ), lactic dehydrogenase isoenzyme V (LAD-5) and N-acetyl-β D-glucosaminidase (NAG), was investigated. LAD-5 appeared particularly valuable for the early detection of upper UTI. However, the overall diagnostic accuracy appeared to be further strengthened using, besides LAD-5, one additional variable. A set of simple and noninvasive biochemical tests on urine samples can reliably help to identify the site of UTI.