This study evaluated detection of ventilatory threshold (VT) in adolescents with mental retardation (MR) (17 with MR, 13 with MR and Down Syndrome (DS), mean age 17.5 years). Participants performed an individualized treadmill VO2peak test. Two evaluators reviewed the same VT plots 6 weeks apart, using five different methods. VE vs. time elicited the most detectable cases (83%), but significantly fewer youth with DS exhibited a detectable VT using any combination of methods (62% vs. 100%). Only VE vs. time yielded an acceptable detection rate, although this may have been influenced by the protocol used. Intra-evaluator correlation coefficients ranged from 0.91–0.97, and interevaluator reliability coefficients ranged from 0.81–0.93. These findings suggest determination of VT is difficult in this population when using an individualized treadmill protocol, especially in adolescents with DS.