The aim of this study is to determine the efficacy of FITNET (web-based cognitive behavioural treatment) for adolescents with Chronic Fatigue Syndrome (CFS) in The Netherlands. The second goal of the study is to establish predictors of outcome. It is very important to know the characteristics of patients who will benefit from Cognitive Behavioural Treatment (CBT) and who will not. Possible predictors of outcome are: age, depression, anxiety, fatigue of the mother, parental bonding, self-efficacy, body consciousness of child and mother, physical activity (Actometer).

All participants will be randomized to one of the two treatment arms:

1. Intervention with web-based cognitive behavioral treatment
2. Usual care

The duration of the cognitive behavioural program is limited to 6 months. The adolescents who have been assigned to the usual care will get the opportunity to attend the program after these 6 months. The total follow-up time is 12 months after the start of the web-based program.

The web-based program is developed for both the adolescents and the parents. The program consists of two parts, a psycho-educational part and a cognitive behavioural part consisting of 21 treatment modules. The therapist activates one or more treatment modules per week, dependent on the progress of the participant. Within a treatment module the participant will keep several journals, answer questions and do several assignments. All answers are sent to the therapist, with whom a weekly email contact will be realized.

Efficacy of the web-based program will be determined after 6 months. There will be a follow-up measurement at 12 months.

Inclusion Criteria:

• Adolescents (12 - 18 years) with Chronic Fatigue Syndrome

Exclusion Criteria:

• Score greater than or equal to 44 on the Stait-Trait Anxiety Inventory for Children
• Score greater than or equal to 20 on the Children's Depression Inventory
• No availability of computer and/or internet
• Risk of suicide
• Mental retardation