We estimated the prevalence of obstructive ventilatory impairment and its relationship to sex, age, chronic bronchitis, asthma, and smoking in a cross-sectional epidemiological study. The population consisted of 3119 persons greater than or equal to 16 years listed as patients in our general practice. From this group, 2740 (88%) underwent dynamic spirometry (Vitalograph). The main results showed a high prevalence of obstruction. 26% had MMEF values less than 75%, 8% FEV-1 less than 75% and 5% FEV-1/FVC less than 65%. Obstruction was significantly correlated with old age, chronic bronchitis, asthma, and smoking.