Renal tubular function can be assessed by measuring low molecular weight proteins in urine. In the present study, urinary levels of protein HC, also called alpha 1-microglobulin, were determined in 84 patients, 3-17 years after conduit diversion or continent urinary reconstruction. Increased excretion of protein HC, indicating tubular dysfunction, was found in 30 patients (36%), but was slight in most cases. Among patients with GFR > 45 ml/min/1.73 m2 at latest follow-up, the fall from preoperative GFR was greater in those with elevated levels of protein HC than in those with normal levels (p < 0.01). No permeation of protein HC through the intestinal mucosa in contact with urine could be demonstrated. Urinary protein HC may be a suitable marker for detecting early renal impairment after urinary diversion.