Proteinase inhibitor (PI) phenotyping and clinical investigations were performed on 20 persons in three generations of a family with α₁-antitrypsin deficiency. Two persons were homozygotes and 9 were heterozygotes for the Z allele; one is the first reported IZ phenotype; 11 were common M-types. Both homozygotes and 5 of the heterozygotes, including the IZ individual, had suffered from recurring or chronic respiratory diseases. However, only mild to moderate impairment in lung function tests was observed in some of these patients (DLCO steady state, 3 subjects; FEV₁, 3 subjects; FEF_25–75, 2 subject; elevation of RV, 2 subjects). The rare IZ type, a 35-year-old female, smoker, showed normal lung function except for an elevated RV. Our results indicate that PI deficiency is not necessarily associated with severe lung destruction if noxious inhalants are absent.