The Obstructive Lung Disease in Northern Sweden (OLIN) Studies started in 1985 as an epidemiological project with the aim to detect preventable risk factors for obstructive lung diseases and allergy. In recent years there has been a focus also on obstructive sleep apnoea syndrome (OSAS) and chronic obstructive pulmonary disease (COPD) besides asthma and allergy. The aim of this thesis was to estimate the prevalence and incidence of COPD, risk factors for COPD, and decline in lung function in relation to COPD. The OLIN cohort I (cI) was recruited in 1985/86 and consisted of all 6610 subjects born 1919-20, 1934-35 and 1949-50 in eight geographical areas of Norrbotten. A postal questionnaire survey was performed in 1985/86, 1992 and in 1996. All subjects reporting respiratory symptoms at the questionnaire in 1985/86 were invited to examination in 1986, 1996 and 2002-03. A random sample of 1500 subjects from the participants at the 1996 postal questionnaire survey was invited to examination in 1996 and 2003. The participation rate has been high, ≥85%. The OLIN cohort III (cIII) was recruited in 1992, a postal questionnaire was sent to a random sample of 5681 subjects aged 20-69 years. In 1994/95 a random sample of 970 subjects were invited to examination of whom 666 participated. The prevalence of COPD in the general population sample (cIII) in ages <45 was 4.1%, 11.6%, 9.1%, and 5.1% according to the criteria of BTS1 , ERS2 , GOLD3 , and ATS4 respectively. The corresponding figures in ages ≥45 were 9.7%, 15.4%, 17.1%, and 16.5% respectively. In the age-stratified general population sample (>45 y, cI), the prevalence was 8.1% and 14.3% according to the BTS and GOLD criteria. The prevalence was strongly associated with higher age and smoking but not gender. The prevalence among smokers 76-77 years old was 45% and 50% (BTS and GOLD criteria). A majority of subjects with COPD had respiratory symptoms (in prevalent BTS 94%), most commonly cough and sputum production. Nearly a half of the subjects with COPD had contacted health care due to respiratory complaints other than common colds, but only a minority reported a physician diagnosis relevant for COPD (16% of prevalent COPD according to BTS in cIII, 31% in cI). The 10-year cumulative incidence of COPD (1986-1996) was estimated at 8.2% (BTS) and 13.5% (GOLD) in the symptomatics of cI, associated with higher age and smoking but not gender. Persistent smoking, male gender and reported chronic productive cough were associated with a faster decline in FEV1. Among incident cases of COPD a large proportion (23% of incident BTS) had a rapid decline in FEV1, >90 ml/year, corresponding to a decrease of 28 percent-units of normal value during ten years.The 7-year cumulative incidence of COPD in the random sample of cI (1996-2003) was estimated at 4.9% and 11.0% (NICE guidelines5 and GOLD) and associated with smoking but not gender. The incidence according to GOLD, but not NICE, was associated with increasing age. In multi-variate analysis most respiratory symptoms were markers of increased risk for developing COPD. In conclusion, the prevalence and the incidence of COPD were associated with age and smoking and affected by the use of different spirometric criteria. Respiratory symptoms marked an increased risk for developing COPD. A high proportion of subjects developing COPD had a rapid decline in lung function. Further, there was a substantial underdiagnosis of COPD. 1 British Thoracic Society: FEV1/VC<0.70 & FEV1<80%predicted (pred), 2 European Respiratory Society: FEV1/VC<88%pred in men, <89%pred in women, 3 Global initiative for Chronic Obstructive Lung Disease:FEV1/FVC<0.70, 4 American Thoracic Society: FEV1/FVC<0.75 + symptoms or physician diagnosis, 5 The British National Institute for Clinical Excellence: FEV1/FVC<0.70 & FEV1<80%pred.