intestinal a1-antitrypsin (a1-AT) clearance is used as a measure of protein leakage into the intestinal tract. We measured the intestinal a1-AT clearance and the random fecal a1-AT concentration in normal subjects and in patients with diarrhea and hypoalbuminemia for the three purposes of the followings: the first, to confirm the usefulness of measuring the intestinal a1-AT clearance as a diagnostic tool of protein losing enteropathy, the second, to detetermine the adequate period of stool collection for measuring the intestinal a1-AT clearance, and the third was to find out the relationship between random fecal a1-AT concentration and intestinal a1-AT clearance. In normal control group (n=6), random mean dry fecal a1-AT concentration was 1.1+-0.3mg/g and mean intestinal a1-AT clearance was 15+-3ml/day, intestinal a1-AT clearance was elevated in all patients with diarrhea except chronic pancreatic disease and irritable bowel syndrome. There was a linear correlation between intestinal a1-AT clearance with stool collection for one day and that for two days (r=0.99). Among 16 patients with protein losing enteropathy, dry fecal a1-AT concentration was elevated in 13 patients (81%) but the intestinal a1 -AT clearances did not correlated well with the random fecal a1-AT concentrations (r=0.125), In conclusion. the intestinal A1-AT clearance is a simple and useful method for diagnosing the protein losing entenopathy and random fecal a1-AT concentration is a useful screening test for intestinal protein leakage as a qualitative test. For the definite and quantitative test, intestianl a1-AT clearance is needed and stool collection for only one day is enough for the adequate result.