During the first year of mass screening for alpha1 antitrypsin deficiency, 108,000 newborn Swedish infants were examined. The screening method and criteria used for selecting infants with alpha1 antitrypsin deficiency were reliable. The Pi Z phenotype occurred at a rate of one per 1,433 infants, and the PiZ allele frequency was .026. The Pi- allele was found in one Pi Z and one Pi S infant. The Pi- allele frequency probably was below .001.