From 1970 to 1983, 69 individuals (aged from infancy to 87 years) with homozygous α₁-antitrypsin deficiency (ZZ) were followed up. 12 had liver disease. Amongst the 33 patients with emphysema mean age of onset of dyspnoea in smokers (32 years) was significantly lower than that in non-smokers (51 years). Mean age at death (excluding that from liver disease) was 48 years in smokers and 67 in non-smokers. In non-smokers mean FEV₁ was 77% of that predicted, but in smokers it was only 38% of that predicted. In non-smokers, FEV₁ declined with age, but there was considerable individual variation. In smokers, the decline correlated with the extent of cigarette smoking in pack years. Mean decrease in FEV₁ in non-smokers was abnormally high (80 ml/year), but significantly less than the massive mean decline of 317 ml/year in smokers. In exsmokers the mean rate of decrease was 61 ml/year.