Symptoms are the body's major signaling device, but symptoms often are frustratingly vague, subjective, and ill reported. In addition, challenges attend making decisions about an individual's ability to function based solely on the medical diagnosis, particularly when impairment is subtle or subjective. Chronic fatigue syndrome (CFS) is a model for understanding the limits of the medical approach to determining impairment and disability. A protocol from the Center for Disease Control and Prevention (CDC) is used to define CFS. This protocol is not a list of diagnostic criteria, and some patients with chronic fatigue may not meet the CDC definition. The syndrome reflects dysfunction in the neuroendocrine system similar in some ways to that seen with depressive illness, and most patients seen for this condition fulfill the criteria for depression, which is another illness characterized by fatigue and somatic complaints. Physicians who evaluate and treat patients with subtle persistent illness know that employers and coworkers respond less positively to an employee's needs when illness is subtle than when impairment is obvious. Physicians must be clear about the family, job, and employment risks patients take when they stay out of work for medical reasons; they also must recognize the risk these patients take when they continue to work despite impaired performance.