The purpose of this study was to investigate the reliability of the Aerobic Power Index (API) submaximal cardiorespiratory exercise test, as well as associated variables of oxygen uptake (ml·kg^-1·min^-1) and ratings of perceived exertion (RPE) in cancer patients who are generally unable to complete maximal or lengthy aerobic fitness tests. Twenty male and female participants (11 male; 9 female) aged between 18 and 70 y (mean = 53.28 ± 11. 82 y) were recruited with medical consent within 4 weeks of completing chemotherapy treatment for a lymphohaematopoietic cancer (LHC). Of the twenty recruited participants’ 2 were excluded from analysis due to disease relapse or complications unrelated to testing occurring within the month following testing. Intra-class correlation coefficient (ICC) scores for power output (W·kg^-1) and oxygen uptake (ml·kg^-1·min^-1) were highly reliable (R₁ = 0.96 and 0.96, respectively) and the ICC for RPE was moderately reliable (R₁ = 0.83). Technical error of measurement results for power output (W·kg^-1), oxygen uptake (ml·kg^-1·min^-1) and RPE were 0.11W·kg^-1, 1.18 ml·kg^-1·min^-1 and 1.0 respectively. A Pearson’s product-moment correlation demonstrated a strong relationship between power output (W·kg^-1) and oxygen uptake (ml·kg^-1·min^-1) for both trials (r = 0.93 and 0.89, respectively). Results demonstrate that the API test is a highly reliable protocol for use with a LHC population and can be considered a clinically feasible, safe and tolerable exercise test.