Inclusion Criteria:

1. Adult patients (≥ 18 years old).
2. Patient understands and signs the Informed Consent.
3. Patients who meet the clinical criteria for the diagnosis of chronic fatigue syndrome as established by the International Chronic Fatigue Syndrome Study Group in 1994 [10].
4. Patients who had a "viral onset" for their CFS.
5. Patients whose CFS symptoms are not spontaneously improving and have plateau for at least 6 months.
6. Patients with "high" antibody titers against HHV-6 IgG ≥ 640, EBV VCA IgG ≥ 640 and detectable EA Ab at 1:160 or HHV-6 IgG ≥ 320 if EBV VCA IgG ≥ 1280 and has detectable EA Ab at 1:160 (measured by the average of a minimum of two time points obtained during screening at least 3 weeks apart).
7. Patient agrees to utilize two reliable methods of contraception combined throughout the study period and for 90 days following discontinuation of the Study Drug.
8. Females of childbearing potential will have a negative pregnancy test at screening.

Exclusion Criteria:

1. Patients who are found to have alternate medical and/or psychiatric causes for their fatigue (see guidelines established by the International Chronic Fatigue Syndrome Study Group in 1994 [1].
2. Patients with history of major depression with psychotic or melancholic features before the diagnosis of CFS or who are found to be actively depressed (major depression with psychotic or melancholic features) by the depression instrument used for the study and by a medical evaluation by a psychiatrist.
3. Patients with other serious co-morbidities which according to the investigator may interfere with the ability of the patient to participate in the study
4. Patients with history of substance abuse in the past year (excluding nicotine and caffeine) or positive urine test for substance abuse.
5. Patients with any other known chronic viral orbacterial infection for which other treatment(s) is(are) available
6. Patients with an active concurrent acute infection
7. Patients with abnormal creatinine clearance (≤60ml/min)
8. Patients with ANC ≤1500 /mm3
9. Patients with Hb ≤ 10 g/dl
10. Patients with platelet count ≤ 100 000/mm3
11. Previous hypersensitivity or contraindication to Valganciclovir/ganciclovir
12. Patients taking other antiviral medications or who have received antiviral medications within the previous three months
13. Patients receiving other experimental therapy
14. Patient is simultaneously participating in another clinical trial
15. Patient requires the use of any prohibited concomitant medications (see Insert on VALCYTE prescribing information).
16. Women in childbearing age considering getting pregnant during the study period
17. Patient is a lactating female who will not discontinue nursing prior to study entry. -