Serum‐trypsin‐inhibitory‐capacity (STIC) and ai‐antitrypsin (AAT) genotypes were evaluated in 83 twins and 112 paired parents‐of‐twins. An increased prevalence (17.0–21.9%) of intermediate AAT deficiency (STIC<0.95 units/ml) was detected in both of these groups as compared to a prevalence of 4.1 % in 1,841 healthy controls. PiS and PiZ molecular variants of AAT were also found more frequently in the twin and parent groups, but this was not statistically significant. Low levels of protease inhibition may enhance fertility and a tendency towards twinning, since proteolytic enzymes are involved in fertilization of ova by sperm and in gametogenesis. Increased fertility and twinning may be heterozygous advantages for A