Alpha-1-antitrypsin (AAT) is the prototypic member of the serine protease inhibitor (serpin) superfamily of proteins, which has a major role in inactivating neutrophil elastase and other proteases to maintain protease-antiprotease balance. In this study, the serum AAT was measured using enzymatic assay in diabetic patients. The serum trypsin inhibitory capacity (s-TIC) was determined in 144 outpatients with uncontrolled insulin-dependent diabetes mellitus (n=47) and non-insulin dependent diabetes mellitus (n=97). The s-TIC values were 1.960+/-0.399, 2.002+/-0.4304 and 2.867+/-0.395 micromol/min/ml in IDDM, NIDDM and healthy subjects, respectively. The diabetics individual had a significantly lower s-TIC than controls (P<0.0001). It was found that there was a negative correlation between the s-TIC and the duration of diabetes (r=-0.5420; P<0.0001). There was no correlation between s-TIC and age of patients (P=0.865). Thus, diabetes is associated with reduced trypsin inhibitory capacity of plasma.