Background. Chronic fatigue and chronic fatigue syndrome are most often encountered in primary care settings. Given the disabling nature of chronic fatigue it may have a substantial impact on service use and costs as well as on employment. This study estimates this impact. Method. Patients presenting to general practitioners with unexplained chronic fatigue were recruited to the study. Service use over a 3 month period was measured and lost employment recorded. These data were used to estimate economic costs. Patients with chronic fatigue syndrome were compared to patients with only chronic fatigue using a multiple regression model with sample differences controlled. Results. The mean total cost of services and lost employment across the sample was £-1906 for the 3-month period with formal services accounting for 9.3 % of this figure. Service use was higher for patients with chronic fatigue syndrome compared to those with chronic fatigue alone. Total 3-month costs were on average higher for chronic fatigue syndrome (£3515 v. £1176) but when sample differences were taken account of the mean difference was reduced to £1406 (P=0.086). Over 90% of the cost was accounted for by care provided by friends and family members and by lost employment. Patients with dependants had significantly higher costs than those with none and costs were also significantly higher for greater levels of functional impairment. Conclusion. Chronic fatigue imposes substantial economic costs on society, mainly in the form of informal care and lost employment. Treatments need to be developed which recognize these impacts.