To examine the relative risk for the development of obstructive lung disease in persons heterozygous for alpha1-antitrypsin deficiency, we determined protease inhibitor phenotypes in 2944 subjects in a general community population. Phenotype M was found in 89.5 per cent, MS in 7.1 per cent, and MZ in 3.0 per cent of the population. There were two persons of phenotype Z and six of phenotype SZ. The study also included respiratory questionnaires and spirometry. There were no statistically significant differences in the prevalence of respiratory symptoms and diagnoses or of ventilatory impairment among the three major phenotype groups (M, MS and MZ), nor were there differences in the rates of deterioration of function with age or smoking. Consequently, we do not consider population screening for heterozygous alpha1-antitrypsin deficiency to be worthwhile.