A series of 111 index subjects with chronic obstructive pulmonary disease (COPD) who had forced expiratory volume in 1 second (FEV1) of 70% or less of that predicted were matched on the basis of age, sex, occupation, and smoking history with control subjects who had an FEV1 of 85% or more of that predicted. Index and control subjects with seasonal or reversible airway disease were excluded. Men outnumbered women by a ratio of 4.5 to 1. Thirty-five percent of the women and 2% of the men were nonsmokers (0 pack-years). There were three PiZ phenotypes in the index group (two nonsmokers) and none in the controls. PiMZ phenotypes in the index group outnumbered those in the controls by 8 to 5. Host factors that might be important in these closely matched pairs were sought by history, physical examination, and a large battery of laboratory tests. A standard respiratory questionnaire revealed the anticipated significantly higher frequency of cough, phlegm, noisy respiration, and all grades of dyspnea in index subjects. Previous lower respiratory tract infections also were more frequent in index subjects than in controls. There were no detectable differences between groups in the frequency of upper airway infections, nasal polyps, sinus surgery, or reported allergy to any substance. If the British Medical Research Council's definition of chronic bronchitis were applied to our study, about two-thirds of our index subjects and almost one-third of our controls would be considered to have chronic bronchitis. Pack-years of smoking were not significantly associated with the amount and duration of cough and expectoration in male or female index subjects or controls. Significant differences between index and control groups on physical examination included the audible forced expiratory flow time over the trachea, the estimated maximal midexpiratory flow, breath sounds, rales, and total excursion of the hemidiaphragms. An endocrine questionnaire and measurement of blood sex hormones did not give any clues as to the propensity of males to develop COPD. Women with airway obstruction similar to that of men had histories of significantly fewer pack-years than did the men, and there was a much larger proportion of women who never smoked. Further studies, specifically on genetic and immunologic characteristics, are under way to identify potential host factors.