Current approaches to the diagnosis and assessment of Chronic Fatigue Syndrome (CFS) rely primarily on scales that measure only the occurrence of various symptoms related to CFS. Such approaches do not provide information on either the severity of symptoms or on fluctuations in symptom severity and activity level that occur over time. As a result, these measures do not reflect the complexities and the interrelations among symptoms. By obscuring the fluctuating nature of CFS and its high variability, current assessment procedures may prevent health care professionals from understanding the complexities of this disease. The present study provides two CFS case studies to illustrate the advantages of using self-reporting rating scales in combination with a device used to measure the frequency and intensity of activity. The implications of this assessment system, which captures the symptom dynamics and variability involved in CFS, are discussed.