alpha 1-antitrypsin (alpha 1AT) of the Pi type Z is associated with two diseases: pulmonary emphysema and cirrhosis of the liver. We report 23 families with both parents heterozygous for the PiZ allele, characterized from our own analysis and from world literature sources. All families were identified through members expressing disease. From the extended pedigrees, 18 backcross families (parents with Pi types MM and MZ) were identified. Analysis of the backcross families reveals a significant increase in Pi MZ offspring (.73) among families where the male is heterozygous. The distortion is not detected among families where the female is heterozygous. Among the matings where both parents are heterozygous, we found 0.43 Pi ZZ from families where one or more members expressed hepatic cirrhosis, and 0.40 Pi ZZ for total families studied. This contrasts to the 0.25 Pi ZZ expected, but is consistent with the distortion observed in backcross matings. The implications of various statistical approaches are discussed, and we point out why our findings differ from previous reports. We suggest a possible biological explanation residing in the fertilization process.