Many overweight and obese individuals have difficulty in performing aerobic fitness tests that require maximal exertion. Therefore, the purpose of this study was to investigate the reliability of the Aerobic Power Index (API) submaximal aerobic exercise test, as well as associated variables consisting of oxygen uptake (ml kg(-1) min(-1)) and ratings of perceived exertion (RPE) in an obese population. Twenty-two obese subjects (3 mates and 19 females) aged between 36 and 68 years (mean = 45.7 +/- 9.8 years), with BMI's ranging between 30.45 and 39.18 kg m(-2) (mean = 33.63 +/- 2.6), participated in two trials that was performed on the same weekday at the same time, but 1 week apart. Intraclass correlation coefficient (ICC) scores for power output (W kg(-1)) and oxygen uptake O-2 (ml kg(-1) min(-1)) were highly reliable (R-1 = 0.95 and 0.96, respectively), while the ICC score for RPE was moderately reliable (R-1 = 0.88). Technical error of measurement results for power output (W kg(-1)), O-2 (ml kg(-1) min(-1)), and RPE were 0.07 W kg(-1), 0.71 ml kg(-1) min(-1), and 0.92, respectively. Additionally, a Pearson product-moment correlation coefficient demonstrated a strong relationship between power output (W kg(-1)) and O-2 (ml kg(-1) min(-1)) for trial 1 (r=0.90) and for trial 2 (r=0.89). Results from this study demonstrated that the API submaximal aerobic exercise test is a highly reliable protocol for use in an obese population, and can be considered as an alternative exercise test to other submaximal cycle tests. (c) 2006 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved.