Use of maximal aerobic exercise testing in a chronically ill population may not only deter potential subjects from participating in trials, or returning for repeat trials, but may also result in the exacerbation of symptoms related to CFS. The Aerobic Power Index represents a submaximal exercise test that forms the aerobic component of the Tri-Level Fitness Profile. This incremental bike test has a predetermined termination point based on a target heart rate (THR) of 75% of age predicted heart rate maximum, making successful completion of the test more likely in chronically ill subjects. The aim of this study was to determine reliability of the Aerobic Power Index in 20 CFS subjects. Results for the 17 subjects who reached THR for both trials, demonstrated high reliability for watts per kilogram and oxygen uptake (ml*kg-1*min-1), as demonstrated by an intraclass correlation coefficient (ICC) of .97 and .91, respectively, while RPE resulted in moderate reliability (ICC = .87). The results of this study indicate that the Aerobic Power Index is a reliable submaximal test for use in a CFS population.