Purpose: This study compared maximal oxygen uptake (VO2max) between estimated and directly measured VO2max values obtained during a maximal cycle ergometer test in breast cancer survivors. Methods: Nine women (50±6 years) diagnosed with early stage breast cancer (BCS) who had completed all primary cancer treatments within the past 3 to 6 months and nine age, weight, and fitness level matched women (59±5), with no history of cancer participated in the study. All subjects performed a VO2max test on an electronically-braked cycle ergometer. Using results of the test, an estimated VO2max was calculated, then compared to the directly measured VO2max obtained during the test using a dependent samples t-test. Results: Significant difference (p=0.01) was observed between directly measured (18.1±2.7ml/kg/min) and estimated (16.3±3.6ml/kg/min) VO2max values in BCS. Conclusion: Estimated VO2max calculated from a submaximal cycle ergometer test underestimates VO2max when compared to directly measured VO2max.