This study was conducted to determine the effects of a leg strengthening program on the 600 yd run/walk time of adolescents with intellectual disabilities (Fernhall & Pitetti, 2000). A single subject ABAB applied intervention analysis was replicated for each of 7 high school participants. Three males and 4 females were tested five times on the 600 yd run/walk and on a CYBEX II isokinetic dynamometer for peak torque, peak force, and average force in each leg. BMI was also calculated and testing was conducted before and every 9 weeks following the two no treatment and two treatment phases. Between weeks 2 and 9 and weeks 20 and 27, the participants received no resistance or cardiorespiratory endurance (CRE) training. During weeks 11 and 18 and 29 and 36, the participants performed only leg strengthening resistance training exercises twice a week. All participants improved in leg strength as a result of the resistance training program in the amount of weight lifted and number of repetitions performed, as evidenced by the improvement on individual progress cards. In addition, varying amounts of improvements in peak torque, peak force and average force in extensors or flexors of either leg were seen in different participants following the leg strengthening programs. After Treatment 2, there was a sizeable mean decrease in BMI of 1.76 kg/m² and a small mean decrease of 18 s in the 600 yard run/walk. Consistent with previous studies, the muscular strength and CRE of these participants with intellectual disabilities were considerably lower than peers without intellectual disabilities. Within the limitations of this study, it can be concluded that a progressive resistance training program can increase the leg strength of high school students with intellectual disabilities in as little as 2 days a week for 8 weeks. Although strong relationships were seen in BMI and leg strength and BMI and 600 yd run/walk, reliable relationships were not found between leg strength and 600 yd run/walk.