Background: Alpha 1 antitrypsin (AT) deficiency is a hereditary disorder leading to the defective defence system against neutrophil elastasis in lung and accumulation of insoluble heterodimer AT molecules in hepatocytes. Knowledge of the prevalence of AT deficiency in each country is important to organize the public health policy. The aim of this study is to determine the prevalence of AT deficiency in Turkish population and to define the cutoff value of AT level in serum to detect heterozygous AT deficient subjects. Materials and Methods: Serum samples from 1,203 healthy blood donors were used, attending the Blood Bank of Hacettepe Medical Faculty. Isoelectric focusing method for determining PIM, PIS, and PIZ alleles and rate immune nephelometry for measuring the level of AT in serum were used. Results: Out of 1,203 healthy blood donors enrolled, 1,164 (% 96.8) had normal variant PI MM allelee, 9 (% 0.7) PI MZ, 7 (% 0.6) PI MS, 6 (% 0.5) MF, and 17 (% 1.4) PI M? (unidentified variants with existing standards). Most individuals (89.6%) with low AT level (cutoff < 100 mg/dl) in serum were positive for PI MM allele. The cutoff value to investigate PI MZ was 100.5 mg/dl, which had PPV and NPV of 5.0 and 99.9%, respectively. AT deficiency is a rare hereditary disorder in asymptomatic healthy Turkish blood donors. Although the cutoff value of 100.5mg/dl for AT level in serum was able to detect heterozygous AT deficiency in the healthy population, this finding should be conformed to case-control studies. J. Clin. Lab. Anal. 25: 296-299, 2011. (C) 2011 Wiley-Liss, Inc.