Chronic exposure to carbon monoxide produces a clinical syndrome that is often overlooked because of obscure symptomatology, a range of presentations, and lack of awareness of the problem. To help physicians recognize and treat the chronic carbon monoxide exposure syndrome, the authors present its objective symptomatology, an approach to diagnosis emphasizing neuropsychological tests, a treatment protocol, and theoretical considerations for the mechanism of hyperbaric oxygen's therapeutic action. For elucidation, eight patients treated in the hyperbaric chamber at a tertiary care facility are described. Diagnosis can be facilitated by recognizing the syndrome based on the patient's history, as well as physical and neuropsychological examinations, with emphasis on identifying potential sources of carbon monoxide exposure. The evaluation and treatment protocols presented, though still experimental, show promise for improving functional, cognitive, and psychiatric capacities.