Background : To determine the prevalence of chronic fatigue syndrome and idiopathic chronic fatigue in Korea and to describe demographic, clinical, and psychological differences among those with chronic fatigue syndrome (CFS), those with idiopathic chronic fatigue, and healthy controls. Methods : 1,526 persons aged 18-76 years who visited Korea university hospital health management center for general check-up between December 1998 and August 1999 were participated in the study. The questionnaire made according to the Centers for Disease Control and Prevention criteria was administered to the recruited persons and patients with chronic fatigue syndrome were diagnosed by questionnaire, physical examination and laboratory tests. The Korean version of the Center for Epidemiological Studies-Depression Scale (CES-D) was used to assess depression. Results : Of the 1,526 persons studied, 433 (29.4 %) reported severe fatigue lasting at least 6 months. Of the 202 persons with unexplained chronic fatigue, 31 persons (2.0% of the study population) were classified as CFS cases. The prevalence of CFS was 2.81% in women, 1.49% in men respectively (p<0.05). When CES-D cut-off score of 25 was used, 30.43% of persons with CFS and 5.93% of persons without chronic fatigue had scores suggestive of depression. CFS patients had higher mean scores on CES-D than persons without chronic fatigue (p<0.05). Conclusion : Persons who met the criteria for chronic fatigue syndrome were found in 2.0%. The prevalence of chronic fatigue syndrome in our study were high, compared with previous studies in other countries. CFS patients had higher mean scores on CES-D than persons without chronic fatigue.(Korean J Med 59:529-534, 2000)