Chronic fatigue syndrome (CFS) presents with fatigue, low motivation, diminished mood, and reduced activity, all symptoms having extensive diagnostic overlaps with depression. Studies have linked chronic viral infections with CFS, and antiviral therapy has effectively treated CFS in adult patients. In a retrospective case series, 15 adolescents and preteens referred to the author for treatment-resistant depression or mood disorder were evaluated and found to have met the Fukuda diagnostic criteria for CFS. While a subset (4/15) had been diagnosed in the past with CFS, the majority had a current diagnosis of depression or a mood disorder. The Diagnostic and Statistical Manual-IV Text Revision (DSM-IV TR) criteria for depression were not met in all patients, although 3 cases of mood disorder not otherwise specified (MD-NOS) and 1 case of Tourette syndrome (TS) plus MD-NOS were diagnosed. Baseline scores on the Children's Depression Inventory (CDI) were below the cutoff for depression in all but 1 patient. Baseline self-assessment scales for CFS or fatigue were obtained and sleep was evaluated with sleep logs. All patients were treated subsequently with valacyclovir, with 93% having a positive response. At the end of treatment, scores on fatigue self-assessment scales improved significantly (P < .001). Vigor subscale scores also improved significantly (P < .001). Some patients experienced complete resolution of symptoms. Although not every patient was tested, available laboratory testing revealed increased counts of natural killer (NK) cells and decreased human herpesvirus 6 (HHV-6) antibody titers in all patients who responded to valacyclovir. This article discusses the significance of infectious agents in the pathogenesis of psychiatric symptoms. The study's data support an intriguing hypothesis that a portion of treatment-resistant depression in fact may be undiagnosed CFS or other chronic viral infection.