The purpose of this study was to compare cardiovascular fitness to leg strength of young adults (mean age = 25.2 yr) with mental retardation (MR) with and without Down syndrome (DS) and to determine whether a relationship exists. Thirteen adults (9 males, 4 females) with DS and 24 adults (16 males, 8 females) with MR but without DS participated in this study. Cardiovascular capacity (VO2peak) for each subject was determined by a treadmill test (GXT) and isokinetic knee flexion and extension strength (peak torque and average power) were determined by isokinetic dynamometry. Results generally demonstrated significant positive relationships (P < 0.05) between VO2peak and isokinetic leg strength (r = 0.61), with the relationship being substantial mainly for subjects with Down syndrome (r = 0.84). The results of this study suggest that leg strength may be an important contributor to VO2peak for persons with mental retardation.