This study was supported in part by a research fund provided by Woodward State Hospital School, Woodward, IA. Although our society has recognized the need of the "normal" population for physical exercise and wellness, little attention has been given to the physical fitness needs of persons with mental retardation. Physical therapy intervention for these individuals is generally restricted to medical problems, usually of orthopaedic or neuromuscular origin, with little emphasis on cardiovascular function. The purpose of this study was to evaluate the effects of an individualized treadmill exercise training (TET) program on the cardiovascular fitness (CVF) of six adults with mental retardation in an institutional setting. A multiple group by time cross-over design was used. The subjects were randomly assigned to either an experimental group (N = 3) or control group (N = 3) after an 8-week orientation period. After completion of baseline measurements, the experimental group underwent 12 weeks (Period I) of progressive TET exercise five times weekly, while the control group received no structured exercise training during this period. Treadmill graded exercise testing was performed every 4 weeks on both groups. After completion of Period I, the control group was put on a TET program similar to the one performed by the experimental group, and the experimental group underwent a maintenance TET program, with exercise two times weekly. Results indicated reduced heart rate (HR) for standardized submaximal treadmill workloads and significantly increased estimated peak oxygen uptake (VO2) (38%) and heart rate (10%) subsequent to 12 weeks of TET. Low frequency TET was effective in maintaining CVF. Treadmill exercise training was found to be a safe exercise modality, and compliance to TET was good among all subjects. In conclusion, TET appears to be an effective modality in improving and maintaining CVF of adults with mental retardation in an institutional setting. Additional research with more subjects is recommended to confirm these results. J Orthop Sports Phys Ther 1992;16(5):220-228.