The Fennell Phase Inventory (FPI) is an instrument designed to measure phases of the illnesses known as Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS). The current study explored how the FPI was related to physical and psychological functioning as well as coping style. Based on FPI scores, 111 adults with ME/CFS were placed in one of three groups: crisis, stabilization, or resolution. Results showed that the crisis group demonstrated significantly worse functioning than at least one other group for depression, quality of life, mental functioning, anxiety, and self-efficacy; and utilized less adaptive coping styles. These results indicate that patients with ME/CFS who are in the crisis phase tend to experience more severe psychological and physical symptoms and utilize poorer coping strategies. Those in the resolution phase maintain the most adaptive coping strategies. Implications for these findings are discussed.