530 tionnaire were interviewed by trained nurses. The question- naire survey was conducted in two stages. In the first stage, the 1,648 participants were questioned on their socio-demo- graphic characteristics and fatigue. Socio-demographic char- acteristics included age, gender, educational qualification, employment, and marriage as well as habits such as drinking and smoking. In addition, the questionnaire included ques- tions on present and past history of illness and medicines being taken currently. Whether the subject was fatigued or not was determined by the question of ‘Have you ever felt fatigue as disabling your everyday life substantially during the last one month?’ and those who replied ‘Yes’ were directed to a ques- tion how long the fatigue had lasted. The period of fatigue was divided into ‘1-6 months’ and ‘Over 6 months.’ In the second stage, another questionnaire was conduced on 139 patients with CF who replied that their fatigue had contin- ued for over 6 months. The questionnaire was composed of questions on the symptoms of CF based on the case defini- tion of CFS and another eight symptoms, questions to screen depression, and questions to measure the intensity of stress. The questions related to CFS included the symptoms of CF and another eight symptoms based on the case definition of CFS proposed by CDC in the U.S. in 1994 (2). Depression was screened by the Zung’s self-rating depression scale (8). The intensity of stress was measured using BEPSI-K (9). Classification of subjects The 1,648 subjects participating in the study were classified into ‘No fatigue,’ ‘Prolonged fatigue (fatigue for 1-6 months)’ and ‘CF (fatigue for over 6 months)’ according to whether they had fatigue and how long the fatigue had lasted. Because fatigue for more than one month was clinically significant, the patients complained of fatigue for less than one month were classified as ‘No fatigue’ (10). The physicions evaluated the CF patients through history taking, physical examina- tions, and laboratory tests, and determined whether the cause of CF could be medically explained or not. Laboratory tests- complete blood count with differential, blood chemistry (liver function test, electrolytes, and lipid profiles), thyroid func- tion test, urinalysis, chest radiography, etc.-were selected with reference to the tests for CF patients recommended by CDC in 1994 (2). In addition, additional tests were performed as necessary. Through this process, CF patients were further categorized into four sub-groups. They were categorized as ‘Explained CF’ if the cause of CF could be medically explained, or as ‘Unexplained CF’ if the cause of CF could not be med- ically explained. ‘Explained CF’ was divided into ‘Physical CF’ and ‘Psychological CF’ according to the major cause of CF. If there were two or more causes that could explain CF, the most relevant single cause was considered to be the major problem associated with CF by physicians’ decision. The sub- jects with unexplained CF who met the case definition crite- ria for CFS were classified as CFS and the others of unexplained C.H. Kim, H.C. Shin, C.W. Won CF did not meet the criteria were classified as idiopathic chron- ic fatigue (ICF) (2). The classification of CF patients and diag- nosis of CFS (including ICF) were made by each physician after evaluating the CF patients at least three times. Analysis The point prevalence and 95% confidence interval (CI) of CF and CFS were calculated by the percentages of patients classified into CF and CFS among the 1,648 participants. Because the 314 patients who did not participate in the study were not statistically different from the 1,634 participants in terms of gender and age, they were excluded from analy- sis. We also estimated the age-sex adjusted prevalence of CF and CFS based on the Korean population distribution in 2000. We estimated the point prevalence of prolonged fatigue and CF according to the age and sex distribution. Comparative multivariate analysis was done on socio-demographic char- acteristics among the three groups-no fatigue, prolonged fatigue, and CF. Major causes of explained CF were identi- fied and their frequencies were counted. In addition, the four groups of CF were compared with regard to their socio-demo- graphic characteristics, the intensity of stress, the frequency of depression, etc. The frequencies of the CF symptoms and another eight symptoms included in the case definition of CFS were comparatively analyzed. The level of statistical sig- nificance was set at 5%, and SAS 6.12 for Windows was used for analyses. RESULTS Among the 1,648 subjects, 348 complained of prolonged fatigue, showing a point prevalence of 21.1% (95% CI 19.1- 23.1%), and 139 of CF, showing a point prevalence of 8.4% Table 1. Point prevalence and age-sex adjust prevalence of chronic fatigue and chronic fatigue syndrome among patients aged ≥18 yr who attended to community-based eight primary care clinics in Korea (n=1,648) No. Prevalence (95% CI) Age-sex adjusted prevalence (95% CI) Prolonged fatigue Chronic fatigue Explained CF Physical CF Psychological CF Unexplained CF CFS ICF 348 21.1% (19.1-23.1%) 23.1% (21.0-25.2%) 139 8.4% (7.1-9.7%) 9.4% (7.9-1.09%) 112 6.8% (5.6-8.0%) 8.7% (7.2-10.1%) 48 2.9% (2.1-3.7%) 3.1% (2.2-4.0%) 64 3.9% (3.0-4.8%) 4.4% (3.3-5.4%) 27 1.6% (1.0-2.2%) 1.9% (1.2-2.6%) 10 0.6% (0.2-1.0%) 0.7% (0.3-1.2%) 17 1.0% (0.5-1.5%) 1.2% (0.6-1.8%) CI, Confidence Interval; CF, Chronic Fatigue; Physical CF, CF that the major cause that could explain chronic fatigue were physical problems; Psychological CF, CF that the major cause that could explain chronic fatigue were psychological problems; CFS, Chronic fatigue syndrome; ICF, Idiopathic chronic fatigue.