PURPOSE: To evaluate the potential benefit of immunologic therapy with dialyzable leukocyte extract and psychologic treatment in the form of cognitive-behavioral therapy (CBT) in patients with chronic fatigue syndrome (CFS). PATIENTS AND METHODS: Immunologic and psychologic treatments were administered to 90 adult patients who fulfilled diagnostic criteria for CFS in a double-blind, randomized, and placebo-controlled study. A four-cell trial design allowed the assessment of benefit from immunologic and psychologic treatment individually or in combination. Outcome was evaluated by measurement of global well-being (visual analogue scales), physical capacity (standardized diaries of daily activities), functional status (Karnofsky performance scale), and psychologic morbidity (Profile of Mood States questionnaire), and cell-mediated immunity was evaluated by peripheral blood T-cell subset analysis and delayed-type hypersensitivity skin testing. RESULTS: Neither dialyzable leukocyte extract nor CBT (alone or in combination) provided greater benefit than the nonspecific treatment regimens. CONCLUSIONS: In this study, patients with CFS did not demonstrate a specific response to immunologic and/or psychologic therapy. The improvement recorded in the group as a whole may reflect both nonspecific treatment effects and a propensity to remission in the natural history of this disorder.