To study further the role of intermediate alpha 1-antitrypsin (AAT) deficiency in chronic obstructive pulmonary disease (COPD), AAT Pi-types and serum-trypsin-inhibitory-capacity (STIC) were measured in 965 patients with COPD. Heterozygosity of the Z variant was the major cause of intermediate AAT deficiency (primarily the MZ phenotype), accounting for 8.0 percent of the patients compared to 2.9 percent of control subjects (p less than .0005). ZZ homozygosity was detected in 1.9 percent of the patients, compared to 0.04 percent of control studies performed by others (none was present in our own control group of 1,380 subjects). The mean age for MS or MZ patients did not differ from that of the COPD patients as a whole, whereas the ZZ homozygotes were younger (55.9 +/- 9.8 vs 65.3 +/- 7.5 years). These results resemble those of a previous study in 66 male veterans with pulmonary emphysema suggesting that the MZ phenotype, or intermediate AAT deficiency in general, probably does predispose to the development of COPD. However, the prevalence of AAT deficiency in COPD patients is small (approximately 10 percent). The number with an MS phenotype was not increased in this group of COPD patients.