KLIKA, R. J., K. E. CALLAHAN, and K. S. GOLIK. Exercise Capacity of a Breast Cancer Survivor: A Case Study. Med. Sci. Sports Exerc., Vol. 40, No. 10, pp. 1711-1716, 2008. Purpose: The processes by which we assess, create an individualized exercise program, and monitor training of a breast cancer survivor who was participating in a fitness plan during and after surgery, chemotherapy, and radiation treatments were examined over a 391-d period. Case Study: A 57-yr-old female was diagnosed with stage I breast cancer (similar to 1.2 cm diameter, estrogen positive, HER2/neu negative) with no lymph node involvement. After lumpectomy and axillary node dissection, the client completed chemotherapy treatment (cyclophosphamide, methotrexate, 5-fluorouracil (CMF)) followed by 33 bouts of radiation therapy. Assessment (body composition, (V)over dotO(2max), lactate threshold, pulmonary function testing) was measured 4 d postdiagnosis and 2 months after treatments had ended. The client kept a daily log of exercise, average heart rate, and rating of perceived exertion in each exercise session. Results: Over 391 calendar days, the client exercised 343 days (88%) and completed 424 exercise sessions. The client's body composition (15.1%) and body weight (41.4 kg) remained stable for the entire period. There was a significant decrease in (V)over dotO(2max) (-7.8%) before and after treatment (56.4 to 52.0 mL.kg(-1).min(-1), respectively). During the treatment phase (6.8 months), the client averaged 1.19 exercise sessions per (lay, with an average duration of 48.1 +/- 25.2 min at approximately 57% of (V)over dotO(2max)(similar to 32 mL.kg(-1).min(-1)). Posttreatment (similar to 6.2 months), the client averaged 1.32 exercise sessions per day, with an average duration of 69.3 +/- 38.5 min at approximately 59.6% (V)over dotO(2max) (similar to 31.2 mL.kg(-1).min(-1)). Pre- and posttreatment exercise durations were significantly different (P < 0.01). Conclusions: A cancer Survivor who engaged in a medically supervised and proactive fitness plan starting from the day of diagnosis maintained a realistic level of physiologic function during and after cancer treatment.